Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.3847/2041-8213/ad93d2

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Astrophysical Journal  

We present 850 μm polarization observations of the IC 348 star-forming region in the Perseus molecular cloud as part of the B-fields In STar-forming Region Observation survey. We study the magnetic properties of two cores (HH 211 MMS and IC 348 MMS) and a filamentary structure of IC 348. We find that the overall field tends to be more perpendicular than parallel to the filamentary structure of the region. The polarization fraction decreases with intensity, and we estimate the trend by power law and the mean of the Rice distribution fittings. The power indices for the cores are much smaller than 1, indicative of possible grain growth to micron size in the cores. We also measure the magnetic field strengths of the two cores and the filamentary area separately by applying the Davis-Chandrasekhar-Fermi method and its alternative version for compressed medium. The estimated mass-to-flux ratios are 0.45-2.20 and 0.63-2.76 for HH 211 MMS and IC 348 MMS, respectively, while the ratios for the filament are 0.33-1.50. This result may suggest that the transition from subcritical to supercritical conditions occurs at the core scale (∼0.05 pc) in the region. In addition, we study the energy balance of the cores and find that the relative strength of turbulence to the magnetic field tends to be stronger for IC 348 MMS than for HH 211 MMS. The result could potentially explain the different configurations inside the two cores: a single protostellar system in HH 211 MMS and multiple protostars in IC 348 MMS.

DOI： 10.3847/1538-4357/ad88ed

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Authorship：Last author   Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.3847/1538-4357/ad77ac

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DOI： 10.3847/1538-4357/ad6763

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DOI： 10.1093/mnras/stae1829

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DOI： 10.3847/1538-4357/ad3638

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DOI： 10.3847/1538-4357/ad2f9a

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.3847/1538-4357/ad1990

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DOI： 10.1093/pasj/psad081

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.3847/1538-4357/ad165b

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Authorship：Last author   Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.3847/1538-4357/ad072a

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1093/mnras/stad3147

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1093/pasj/psad040

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Publisher：Astrophysical Journal  

Numerous exoplanets with masses ranging from Earth to Neptune and radii larger than Earth have been found through observations. These planets possess atmospheres that range in mass fractions from 1% to 30%, reflecting the diversity of atmospheric mass fractions. Such diversities are supposed to be caused by differences in the formation processes or evolution. Here, we consider head-on giant impacts onto planets causing atmosphere losses in the later stage of their formation. We perform smoothed particle hydrodynamic simulations to study the impact-induced atmosphere loss of young super-Earths with 10%-30% initial atmospheric mass fractions. We find that the kinetic energy of the escaping atmosphere is almost proportional to the sum of the kinetic impact energy and self-gravitational energy released from the merged core. We derive the relationship between the kinetic impact energy and the escaping atmosphere mass. The giant impact events for planets of comparable masses are required in the final stage of the popular scenario of rocky planet formation. We show it results in a significant loss of the atmosphere, if the impact is a head-on collision with comparable masses. This latter fact provides a constraint on the formation scenario of rocky planets with substantial atmospheres.

DOI： 10.3847/1538-4357/ace9ba

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Publisher：Astrophysical Journal  

Secular gravitational instability (GI) is one promising mechanism for explaining planetesimal formation. Previous studies of secular GI utilized a razor-thin disk model and derived the growth condition in terms of vertically integrated physical values such as dust-to-gas surface density ratio. However, in weakly turbulent disks where secular GI can operate, a dust disk can be orders of magnitude thinner than a gas disk, and analyses treating the vertical structures are necessary to clarify the interplay of the midplane dust motion and the upper gas motion. In this work, we perform vertically global linear analyses of secular GI with a vertical domain size of a few gas scale heights. We find that dust grains accumulate radially around the midplane while gas circulates over the whole vertical region. We obtain well-converged growth rates when the outer gas boundary is above two gas scale heights. The growth rates are underestimated if we assume the upper gas to be steady and regard it just as the source of external pressure to the dusty lower layer. Therefore, treating the upper gas motion is important even when the dust disk is much thinner than the gas disk. Conducting a parameter survey, we represent the growth condition in terms of the Toomre Q value for dust and dust-to-gas surface density ratio. The critical dust disk mass for secular GI is ∼10⁻⁴ M <inf>*</inf> for a dust-to-gas surface density ratio of 0.01, a Stokes number of 0.1, and a radial dust diffusivity of 10⁻⁴ c <inf>s</inf> H, where c <inf>s</inf> is the gas sound speed, and H is the gas scale height.

DOI： 10.3847/1538-4357/ace043

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Astrophysical Journal  

Most stars form in multiple-star systems. For a better understanding of their formation processes, it is important to resolve the individual protostellar components and the surrounding envelope and disk material at the earliest possible formation epoch, because the formation history can be lost in a few orbital timescales. Here we present Atacama Large Millimeter/submillimeter Array observational results of a young multiple protostellar system, IRAS 04239+2436, where three well-developed large spiral arms were detected in the shocked SO emission. Along the most conspicuous arm, the accretion streamer was also detected in the SO<inf>2</inf> emission. The observational results are complemented by numerical magnetohydrodynamic simulations, where those large arms only appear in magnetically weakened clouds. Numerical simulations also suggest that the large triple spiral arms are the result of gravitational interactions between compact triple protostars and the turbulent infalling envelope.

DOI： 10.3847/1538-4357/acdd5b

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DOI： 10.3847/1538-4357/acd6f2

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Publisher：Monthly Notices of the Royal Astronomical Society  

Stars in the Galactic disc, including the Solar system, have deviated from their birth orbits and have experienced radial mixing and vertical heating. By performing hydrodynamical simulations of a galactic disc, we investigate how much tracer particles, which are initially located in the disc to mimic newborn stars and the thin and thick disc stars, are displaced from initial near-circular orbits by gravitational interactions with giant molecular clouds (GMCs). To exclude the influence of other perturbers that can change the stellar orbits, such as spiral arms and the bar, we use an axisymmetric form for the entire galactic potential. First, we investigate the time evolution of the radial and vertical velocity dispersion σ<inf>R</inf> and σ<inf>z</inf> by comparing them with a power-law relation of σ ∝ t^β. Although the exponents β decrease with time, they keep large values of 0.3 ∼ 0.6 for 1 Gyr, indicating fast and efficient disc heating. Next, we find that the efficient stellar scattering by GMCs also causes a change in angular momentum for each star and, therefore, radial migration. This effect is more pronounced in newborn stars than old disc stars; nearly 30 per cent of stars initially located on the galactic mid-plane move more than 1 kpc in the radial direction for 1 Gyr. The dynamical heating and radial migration drastically occur in the first several hundred Myr. As the amplitude of the vertical oscillation increases, the time spent in the galactic plane, where most GMCs are distributed, decreases, and the rate of an increase in the heating and migration slows down.

DOI： 10.1093/mnras/stad1612

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DOI： 10.3847/1538-4357/acbea4

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Publisher：Astrophysical Journal  

We investigate the thermal condensation caused by a massive object that passes through the interstellar medium with high velocity, and propose a mechanism for creating a filamentary gaseous object, or interstellar contrail. Our main result shows that a long interstellar contrail can form with a certain parameter; a compact object more massive than 10⁴ M <inf>☉</inf> can make a filament whose length is larger than 100 pc. Observation of interstellar contrails may provide information on the number, masses, and velocities of fast-moving massive objects, and can be a new method for probing invisible gravitating sources such as intermediate-mass black holes.

DOI： 10.3847/1538-4357/acb7ea

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Publisher：Astrophysical Journal  

We investigate the gravitational instability (GI) of dust ring structures and the formation of planetesimals by their gravitational collapse. The normalized dispersion relation of a self-gravitating ring structure includes two parameters that are related to its width and line mass (the mass per unit length). We survey these parameters and calculate the growth rate and wavenumber. Additionally, we investigate the formation of planetesimals by growth of the GI of the ring that is formed by the growth of the secular GI of the protoplanetary disk. We adopt a massive, dust-rich disk as a disk model. We find the range of radii for fragmentation by the ring GI as a function of the width of the ring. The innermost radius for the ring GI is smaller for a smaller ring width. We also determine the range of the initial planetesimal mass resulting from the fragmentation of the ring GI. Our results indicate that the planetesimal mass can be as large as 10²⁸ g at its birth after the fragmentation. It can be as low as about 10²⁵ g if the ring width is 0.1% of the ring radius, and the lower limit increases with the ring width. Furthermore, we obtain approximate formulae for the upper and lower limits of the planetesimal mass. We predict that the planetesimals formed by the ring GI have prograde rotations because of the Coriolis force acting on the contracting dust. This is consistent with the fact that many trans-Neptunian binaries exhibit prograde rotation.

DOI： 10.3847/1538-4357/ac9fd0

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Publisher：Astrophysical Journal  

The angular momentum of molecular cloud cores plays an important role in the process of star formation. However, the time evolution of the angular momentum of molecular cloud cores is still unclear. In this paper, we perform three-dimensional simulations to investigate the time evolution of the angular momentum of molecular cloud cores formed through filament fragmentation. As a result, we find that most of the cores rotate perpendicular to the filament axis. The mean angular momentum of the cores changes by only around 30% during the initial stage of their formation process and then remains almost constant. In addition, we analyze the internal angular momentum structure of the cores. Although the cores gain angular momentum with various directions from the initial turbulent velocity fluctuations of their parent filaments, the angular momentum profile in each core converges to the self-similar solution. We also show that the degree of complexity of the angular momentum structure in a core decreases slightly with time. Moreover, we perform synthetic observations and show that the angular momentum profile measured from the synthetic mean velocity map is compatible with the observations when the filament inclination is taken into account. The present study suggests a theory of core formation from filament fragmentation where the angular momentum structures of the cores are determined by the velocity fluctuation along the filaments and both are compatible with the observations. This theory also provides new insights into the core properties that could be tested observationally.

DOI： 10.3847/1538-4357/aca88d

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Publisher：Astrophysical Journal  

We study the Hii regions associated with the NGC 6334 molecular cloud observed in the submillimeter and taken as part of the B-fields In STar-forming Region Observations Survey. In particular, we investigate the polarization patterns and magnetic field morphologies associated with these Hii regions. Through polarization pattern and pressure calculation analyses, several of these bubbles indicate that the gas and magnetic field lines have been pushed away from the bubble, toward an almost tangential (to the bubble) magnetic field morphology. In the densest part of NGC 6334, where the magnetic field morphology is similar to an hourglass, the polarization observations do not exhibit observable impact from Hii regions. We detect two nested radial polarization patterns in a bubble to the south of NGC 6334 that correspond to the previously observed bipolar structure in this bubble. Finally, using the results of this study, we present steps (incorporating computer vision; circular Hough transform) that can be used in future studies to identify bubbles that have physically impacted magnetic field lines.

DOI： 10.3847/1538-4357/acac81

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Publisher：Astrophysical Journal  

We present 850 μm dust polarization observations of the massive DR21 filament from the B-fields In STar-forming Region Observations (BISTRO) survey, using the POL-2 polarimeter and the SCUBA-2 camera on the James Clerk Maxwell Telescope. We detect ordered magnetic fields perpendicular to the parsec-scale ridge of the DR21 main filament. In the subfilaments, the magnetic fields are mainly parallel to the filamentary structures and smoothly connect to the magnetic fields of the main filament. We compare the POL-2 and Planck dust polarization observations to study the magnetic field structures of the DR21 filament on 0.1-10 pc scales. The magnetic fields revealed in the Planck data are well-aligned with those of the POL-2 data, indicating a smooth variation of magnetic fields from large to small scales. The plane-of-sky magnetic field strengths derived from angular dispersion functions of dust polarization are 0.6-1.0 mG in the DR21 filament and ∼0.1 mG in the surrounding ambient gas. The mass-to-flux ratios are found to be magnetically supercritical in the filament and slightly subcritical to nearly critical in the ambient gas. The alignment between column density structures and magnetic fields changes from random alignment in the low-density ambient gas probed by Planck to mostly perpendicular in the high-density main filament probed by James Clerk Maxwell Telescope. The magnetic field structures of the DR21 filament are in agreement with MHD simulations of a strongly magnetized medium, suggesting that magnetic fields play an important role in shaping the DR21 main filament and subfilaments.

DOI： 10.3847/1538-4357/ac9dfb

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Language：English   Publisher：Astrophysical Journal  

We present and analyze observations of polarized dust emission at 850 μm toward the central 1 × 1 pc hub-filament structure of Monoceros R2 (Mon R2). The data are obtained with SCUBA-2/POL-2 on the James Clerk Maxwell Telescope (JCMT) as part of the B-fields in Star-forming Region Observations survey. The orientations of the magnetic field follow the spiral structure of Mon R2, which are well described by an axisymmetric magnetic field model. We estimate the turbulent component of the magnetic field using the angle difference between our observations and the best-fit model of the underlying large-scale mean magnetic field. This estimate is used to calculate the magnetic field strength using the Davis–Chandrasekhar–Fermi method, for which we also obtain the distribution of volume density and velocity dispersion using a column density map derived from Herschel data and the C¹⁸O (J = 3 - 2) data taken with HARP on the JCMT, respectively. We make maps of magnetic field strengths and mass-to-flux ratios, finding that magnetic field strengths vary from 0.02 to 3.64 mG with a mean value of 1.0 ± 0.06 mG, and the mean critical mass-to-flux ratio is 0.47 ± 0.02. Additionally, the mean Alfvén Mach number is 0.35 ± 0.01. This suggests that, in Mon R2, the magnetic fields provide resistance against large-scale gravitational collapse, and the magnetic pressure exceeds the turbulent pressure. We also investigate the properties of each filament in Mon R2. Most of the filaments are aligned along the magnetic field direction and are magnetically subcritical.

DOI： 10.3847/1538-4357/ac99e0

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Publisher：Astrophysical Journal  

In our previous work (Paper I), we demonstrated that coagulation instability results in dust concentration against depletion due to the radial drift and accelerates dust growth locally. In this work (Paper II), we perform numerical simulations of coagulation instability taking into account effects of backreaction to gas and collisional fragmentation of dust grains. We find that the slowdown of the dust drift due to backreaction regulates dust concentration in the nonlinear growth phase of coagulation instability. The dust-to-gas surface density ratio increases from 10⁻³ up to ∼10⁻². Each resulting dust ring tends to have a mass of ≃0.5 M <inf>⊕</inf> − 1.5 M <inf>⊕</inf> in our disk model. In contrast to Paper I, the dust surface density profile shows a local plateau structure at each dust ring. In spite of the regulation at the nonlinear growth, the efficient dust concentration reduces their collision velocity. As a result, dust grains can grow beyond the fragmentation barrier, and the dimensionless stopping time reaches unity, as in Paper I. The necessary condition for the efficient dust growth is (1) weak turbulence of α < 1 × 10⁻³ and (2) a large critical velocity for dust fragmentation (>1 m s⁻¹). The efficient dust concentration in outer regions will reduce the inward pebble flux and is expected to decelerate the planet formation via the pebble accretion. We also find that the resulting rings can be unstable to secular gravitational instability (GI). The subsequent secular GI promotes planetesimal formation. We thus expect that a combination of these instabilities is a promising mechanism for dust-ring and planetesimal formation.

DOI： 10.3847/1538-4357/ac97e8

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Language：English   Publisher：Astrophysical Journal  

Our previous linear analysis presents a new instability driven by dust coagulation in protoplanetary disks. The coagulation instability has the potential to concentrate dust grains into rings and assist dust coagulation and planetesimal formation. In this series of papers, we perform numerical simulations and investigate the nonlinear outcome of coagulation instability. In this paper (Paper I), we first conduct local simulations to demonstrate the existence of coagulation instability. Linear growth observed in the simulations is in good agreement with the previous linear analysis. We next conduct radially global simulations to demonstrate that coagulation instability develops during the inside-out disk evolution owing to dust growth. To isolate the various effects on dust concentration and growth, we neglect the effects of back-reaction to a gas disk and dust fragmentation in Paper I. This simplified simulation shows that neither back-reaction nor fragmentation is a prerequisite for local dust concentration via the instability. In most runs with weak turbulence, dust concentration via coagulation instability overcomes dust depletion due to radial drift, leading to the formation of multiple dust rings. The nonlinear development of coagulation instability also accelerates dust growth, and the dimensionless stopping time τ <inf>s</inf> reaches unity even at outer radii (>10 au). Therefore, coagulation instability is one promising process to retain dust grains and to accelerate dust growth beyond the drift barrier.

DOI： 10.3847/1538-4357/ac82b4

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Language：English   Publisher：Monthly Notices of the Royal Astronomical Society  

This work assesses the dissipative properties of high-order numerical methods for relativistic hydrodynamics. A causal theory of physical dissipation is included within a finite volume high-resolution shock-capturing framework based on the Israel-Stewart theory to study high-order WENO (weighted-essentially non-oscillatory) schemes for simulating the relativistic Kelvin-Helmholtz instability. We provide an estimation of the numerical dissipation of high-order schemes based on results obtained both with and without physically resolved dissipation and determine an empirical relationship between the numerical dissipation and the grid resolution. We consider the appearance of secondary flow features within the evolution of the Kelvin-Helmholtz instability and determine that they are numerical artifacts-This is partly based on arguments presented in terms of a frame-dependent form of the relativistic Reynolds number. There is a potential advantage of using high-order schemes in terms of their accuracy and computational cost on coarser grid resolutions when directly compared to low-order schemes on a fine grid in the presence of physical viscosity. It is possible to find reasonable agreement between numerical results that employ lower-order schemes using a finer grid resolution and results that employ higher order schemes at a coarser grid resolution when sufficient viscosity is present. Overall, the present analysis gives an insight into the numerical dissipation of high-order shock-wave capturing schemes which can be relevant to computational studies of astrophysical phenomena in the relativistic regime. The results presented herein are problem and scheme-dependent and serve to highlight the different roles of numerical and physical dissipation.

DOI： 10.1093/mnras/stac1741

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Language：English   Publisher：Astrophysical Journal  

Theoretically, misalignment between the magnetic field and rotational axis in a dense core is considered to be dynamically important in the star formation process; however, the extent of this influence remains observationally unclear. For a sample of 32 Class 0 and I protostars in the Perseus Molecular Cloud, we analyzed gas motions using C18O data from the SMA MASSES survey and the magnetic field structures using 850 μm polarimetric data from the JCMT BISTRO-1 survey and archive. We do not find any significant correlation between the velocity gradients in the C18O emission in the protostellar envelopes at a 1000 au scale and the misalignment between the outflows and magnetic field orientations in the dense cores at a 4000 au scale, and there is also no correlation between the velocity gradients and the angular dispersions of the magnetic fields. However, a significant dependence on the misalignment angles emerges after we normalize the rotational motion by the infalling motion, where the ratios increase from 21 to 31 with increasing misalignment angle. This suggests that the misalignment could prompt angular momentum transportation to the envelope scale but is not a dominant factor in determining the envelope rotation, and other parameters, such as mass accretion in protostellar sources, also play an important role. These results remain valid after taking into account projection effects. The comparison between our estimated angular momentum in the protostellar envelopes and the sizes of the known protostellar disks suggests that significant angular momentum is likely lost between radii of ∼1000 and 100 au in protostellar envelopes.

DOI： 10.3847/1538-4357/ac63bc

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Language：English   Publisher：Astronomy and Astrophysics  

Context. Despite recent observational and theoretical advances in mapping the magnetic fields associated with molecular clouds, their three-dimensional (3D) morphology remains unresolved. Multi-wavelength and multi-scale observations will allow us to paint a comprehensive picture of the magnetic fields of these star-forming regions. Aims. We reconstructed the 3D magnetic field morphology associated with the Perseus molecular cloud and compared it with predictions of cloud-formation models. These cloud-formation models predict a bending of magnetic fields associated with filamentary molecular clouds. We compared the orientation and direction of this field bending with our 3D magnetic-field view of the Perseus cloud. Methods. We used previous line-of-sight and plane-of-sky magnetic field observations as well as Galactic magnetic field models to reconstruct the complete 3D magnetic field vectors and morphology associated with the Perseus cloud. Results. We approximated the 3D magnetic field morphology of the cloud as a concave arc that points in the decreasing longitude direction in the plane of the sky (from our point of view). This field morphology preserves a memory of the Galactic magnetic field. In order to compare this morphology to cloud-formation model predictions, we assume that the cloud retains a memory of its most recent interaction. After incorporating velocity observations, we find that the line-of-sight magnetic field observations are consistent with predictions of shock-cloud-interaction models. Conclusions. To our knowledge, this is the first time that the 3D magnetic fields of a molecular cloud have been reconstructed. We find the 3D magnetic field morphology of the Perseus cloud to be consistent with the predictions of the shock-cloud-interaction model that describes the formation mechanism of filamentary molecular clouds.

DOI： 10.1051/0004-6361/202141170

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Language：English   Publisher：Astronomy and Astrophysics  

Context. The interstellar medium is observed to be organized in filamentary structures, and in neutral (H I) and ionized (H II) bubbles. The expanding nature of these bubbles shapes the surrounding medium and possibly plays a role in the formation and evolution of the interstellar filaments. The impact of the expansion of these bubbles on the interstellar medium is not well understood. Aims. Our aim is to describe the kinematics of a filamentary molecular cloud forming high-mass stars and hosting multiple H II regions in order to study the possible environmental impact on the properties of molecular filaments. Methods. We present APEX 13CO and C18O(2-1) mapping observations of the 10 × 50 pc NGC 6334 molecular cloud complex. We investigated the gas velocity structure along and across the 50 pc long cloud and toward velocity-coherent filaments (VCFs). Results. The NGC 6334 complex is observed to have a coherent velocity structure smoothly varying by ∼5 km s-1 over its 50 pc elongation parallel to the Galactic plane. We identify a sample of 75 VCFs in the C18O(2-1) position-position-velocity cube and present the properties of 47 VCFs with a length ³1 pc (five beams). We measure a large number of velocity gradients along the VCFs. The amplitudes of these velocity gradients and the velocity dispersion measured along the crests increase with the column density of the VCFs. We derive the column density and velocity power spectra of the VCFs. These power spectra are well represented with power laws showing similar slopes for the two quantities (with a mean of about -2), although some differ by up to a factor of 2. The position velocity diagrams perpendicular to three VCFs (selected from different physical environments) show the V-shaped velocity pattern corresponding to a bent structure in velocity space with the filament at the tip of the V surrounded by an extended structure connected to it with a velocity gradient. This velocity structure is qualitatively similar to that resulting from numerical simulations of filament formation from large-scale compression from propagating shock fronts. In addition, the radial profiles perpendicular to these VCFs hint to small-scale internal impacts from neighboring H II bubbles on two of them, while the third is mostly unaffected. Conclusions. The observed opposite curvature in velocity space (V- and A-shaped) toward the VCFs points to various origins of large-scale external compressions from propagating H I bubbles. This suggests the plausible importance of multiple H I compressions, separated in space and time, in the formation and evolution of molecular clouds and their star formation history. These atomic compressions due to past and distant star formation events are complemented by the impact of H II bubbles from present time and local star formation activity.

DOI： 10.1051/0004-6361/202141699

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Language：English   Publisher：Astronomy and Astrophysics  

Magnetic fields permeate the interstellar medium and are important in the star formation process. Determining the three-dimensional (3D) magnetic fields of molecular clouds will allow us to better understand their role in the evolution of these clouds and the formation of stars. We fully reconstruct the approximate 3D magnetic field morphology of the Orion A molecular cloud (on scales of a few to ∼100 pc) using Galactic magnetic field models, as well as available line-of-sight and plane-of-sky magnetic field observations. While previous studies identified the 3D magnetic field morphology of the Orion A cloud as an arc shape, in this study we provide the orientation of this arc-shaped field and its plane-of-sky direction for the first time. We find that this 3D field is a tilted, semi-convex (from our point of view) structure and mostly points in the direction of decreasing latitude and longitude on the plane of the sky from our vantage point. The previously identified bubbles and events in this region were key in shaping this arc-shaped magnetic field morphology.

DOI： 10.1051/0004-6361/202243322

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Language：Japanese   Publisher：Astronomy and Astrophysics  

There is growing evidence of the role of hub-filament systems (HFS) in the formation of stars from low to high masses. As of today, however, the detailed structures of these systems are still not well described. Here we study the Mon R2 star-forming region, which has a rich network of filaments joining in a star cluster forming hub, and aim to understand the hub structure and to examine the mass fraction residing in the hub and in the filaments, which is a key factor that influences massive star formation. We conducted a multi-scale, multi-component analysis of the Herschel column density maps (resolution of 18.2′′ or ∼0.07 pc at 830 pc) of the region using a newly developed algorithm getsf to identify the structural components, namely, extended cloud, filaments, and sources. We find that cascades of lower column density filaments coalesce to form higher-density filaments eventually merging inside the hub (0.8 pc radius). As opposed to the previous view of the hub as a massive clump with ∼1 pc radius, we find it to be a network of short high-density filaments. We analyse the orientations and mass per unit length (M/L) of the filaments as a function of distance from the hub centre. The filaments are radially aligned towards the centre of the hub. The total mass reservoir in the Mon R2 HFS (5 pc × 5 pc) is split between filaments (54%), an extended cloud (37%), and sources (9%). The M/L of filaments increases from ∼10 M· pc-1 at 1.5 pc from the hub to ∼100 M· pc-1 at its centre, while the number of filaments per annulus of 0.2 pc width decreases from 20 to two in the same range. The observed radial column density structure of the HFS (filament component only) displays a power-law dependence of NH2 r-2.17 up to a radius of ∼2.5 pc from the central hub, resembling a global collapse of the HFS. We present a scenario where the HFS can be supported by magnetic fields which interact, merge, and reorganise themselves as the filaments coalesce. We plotted the plane-of-the-sky magnetic field line geometry using archival Planck data to support our scenario. In the new view of the hub as a network of high-density filaments, we suggest that only the stars located in the network can benefit from the longitudinal flows of gas to become massive, which may explain the reason for the formation of many low-mass stars in cluster centres. We show the correlation of massive stars in the region to the intertwined network-like hub, based on which we updated the implications of the filaments to clusters (F2C) model for massive star formation.

DOI： 10.1051/0004-6361/202140363

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Language：Japanese   Publisher：Astrophysical Journal  

We present 850 μm polarimetric observations toward the Serpens Main molecular cloud obtained using the POL-2 polarimeter on the James Clerk Maxwell Telescope as part of the B-fields In STar-forming Region Observations survey. These observations probe the magnetic field morphology of the Serpens Main molecular cloud on about 6000 au scales, which consists of cores and six filaments with different physical properties such as density and star formation activity. Using the histogram of relative orientation (HRO) technique, we find that magnetic fields are parallel to filaments in less-dense filamentary structures where NH2<0.93×1022 cm-2 (magnetic fields perpendicular to density gradients), while they are perpendicular to filaments (magnetic fields parallel to density gradients) in dense filamentary structures with star formation activity. Moreover, applying the HRO technique to denser core regions, we find that magnetic field orientations change to become perpendicular to density gradients again at NH2≈4.6×1022 cm-2. This can be interpreted as a signature of core formation. At NH2≈16×1022 cm-2, magnetic fields change back to being parallel to density gradients once again, which can be understood to be due to magnetic fields being dragged in by infalling material. In addition, we estimate the magnetic field strengths of the filaments (B POS = 60-300 μG)) using the Davis-Chandrasekhar-Fermi method and discuss whether the filaments are gravitationally unstable based on magnetic field and turbulence energy densities.

DOI： 10.3847/1538-4357/ac4bbe

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Language：Japanese   Publisher：Astrophysical Journal  

The effects of cosmic-ray diffusion and radiative cooling on the structure of the Galactic wind are studied using a steady-state approximation. It is known that realistic cooling processes suppress the wind from launching. The effects of cosmic-ray diffusion are also supposed to be unfavorable for launching the wind. Both of these effects have not been studied simultaneously in a steady-state approximation of the wind. We find 327,254 solutions of the steady-state Galactic wind and confirm that: the effect of the cosmic-ray pressure depends on the Alfvén Mach number, the mass flux carried by the wind does not depend on the cosmic-ray pressure directly (but depends on the thermal pressure), and the typical conditions found in the Galaxy may correspond to the wind solution that provides metal-polluted matter at a height of ∼300 kpc from the disk.

DOI： 10.3847/1538-4357/ac4110

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Scopus

Language：Japanese   Publisher：Astrophysical Journal  

We present a new instability driven by a combination of coagulation and radial drift of dust particles. We refer to this instability as "coagulation instability"and regard it as a promising mechanism to concentrate dust particles and assist planetesimal formation in the very early stages of disk evolution. Because of dust-density dependence of collisional coagulation efficiency, dust particles efficiently (inefficiently) grow in a region of positive (negative) dust density perturbations, leading to a small radial variation of dust sizes and as a result radial velocity perturbations. The resultant velocity perturbations lead to dust concentration and amplify dust density perturbations. This positive feedback makes a disk unstable. The growth timescale of coagulation instability is a few tens of orbital periods even when dust-to-gas mass ratio is on the order of 10-3. In a protoplanetary disk, radial drift and coagulation of dust particles tend to result in dust depletion. The present instability locally concentrates dust particles even in such a dust-depleted region. The resulting concentration provides preferable sites for dust-gas instabilities to develop, which leads to further concentration. Dust diffusion and aerodynamical feedback tend to stabilize short-wavelength modes, but do not completely suppress the growth of coagulation instability. Therefore, coagulation instability is expected to play an important role in setting up the next stage for other instabilities, such as streaming instability or secular gravitational instability, to further develop toward planetesimal formation.

DOI： 10.3847/1538-4357/ac173a

Open Access

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Scopus

Language：Japanese   Publisher：Astrophysical Journal Letters  

We investigate the internal 3D magnetic structure of dense interstellar filaments within NGC 1333 using polarization data at 850 μm from the B-fields In STar-forming Region Observations survey at the James Clerk Maxwell Telescope. Theoretical models predict that the magnetic field lines in a filament will tend to be dragged radially inward (i.e., pinched) toward the central axis due to the filament's self-gravity. We study the cross-sectional profiles of the total intensity (I) and polarized intensity (PI) of dust emission in four segments of filaments unaffected by local star formation that are expected to retain a pristine magnetic field structure. We find that the filaments' FWHMs in PI are not the same as those in I, with two segments being appreciably narrower in PI (FWHM ratio ≃0.7-0.8) and one segment being wider (FWHM ratio ≃1.3). The filament profiles of the polarization fraction (P) do not show a minimum at the spine of the filament, which is not in line with an anticorrelation between P and I normally seen in molecular clouds and protostellar cores. Dust grain alignment variation with density cannot reproduce the observed P distribution. We demonstrate numerically that the I and PI cross-sectional profiles of filaments in magnetohydrostatic equilibrium will have differing relative widths depending on the viewing angle. The observed variations of FWHM ratios in NGC 1333 are therefore consistent with models of pinched magnetic field structures inside filaments, especially if they are magnetically near-critical or supercritical.

DOI： 10.3847/2041-8213/ac3cc1

Open Access

Web of Science

Scopus

Language：Japanese  

DOI： 10.1093/mnras/stab2748

Web of Science

Language：Japanese  

DOI： 10.1093/mnras/stab2748

Web of Science

Language：Japanese   Publisher：Astrophysical Journal Letters  

Dust growth and its associated dynamics play key roles in the first phase of planet formation in young stellar objects. Observations have detected signs of dust growth in very young protoplanetary disks. Furthermore, signs of planet formation, gaps in the disk at a distance of several tens of au from the central protostar, are also reported. From a theoretical point of view, however, planet formation in the outer regions is difficult due to the rapid inward drift of dust, called the radial drift barrier. Here, on the basis of three-dimensional magnetohydrodynamical simulations of disk evolution with dust growth, we propose a mechanism called the "ashfall"phenomenon, induced by a powerful molecular outflow driven by a magnetic field that may circumvent the radial drift barrier. We find that the large dust that grows to a size of about a centimeter in the inner region of a disk is entrained by an outflow from the disk. Then, large dust decoupled from gas is ejected from the outflow due to centrifugal force, enriching the grown dust in the envelope and eventually falls onto the outer edge of the disk. The overall process is similar to the behavior of ashfall from volcanic eruptions. In the ashfall phenomenon, the Stokes number of dust increases by reaccreting to the less dense disk outer edge. This may allow the dust grains to overcome the radial drift barrier. Consequently, the ashfall phenomenon can provide a crucial assist for making the formation of the planetesimals in outer regions of the disk possible, and hence the formation of wide-orbit planets and gaps.

DOI： 10.3847/2041-8213/ac2b2f

Web of Science

Scopus

Language：Japanese   Publisher：Astrophysical Journal  

We present the four-year survey results of monthly submillimeter monitoring of eight nearby (<500 pc) star-forming regions by the JCMT Transient Survey. We apply the Lomb-Scargle Periodogram technique to search for and characterize variability on 295 submillimeter peaks brighter than 0.14 Jy beam-1, including 22 disk sources (Class II), 83 protostars (Class 0/I), and 190 starless sources. We uncover 18 secular variables, all of them protostars. No single-epoch burst or drop events and no inherently stochastic sources are observed. We classify the secular variables by their timescales into three groups: Periodic, Curved, and Linear. For the Curved and Periodic cases, the detectable fractional amplitude, with respect to mean peak brightness, is ∼4% for sources brighter than ∼0.5 Jy beam-1. Limiting our sample to only these bright sources, the observed variable fraction is 37% (16 out of 43). Considering source evolution, we find a similar fraction of bright variables for both Class 0 and Class I. Using an empirically motivated conversion from submillimeter variability to variation in mass accretion rate, six sources (7% of our full sample) are predicted to have years-long accretion events during which the excess mass accreted reaches more than 40% above the total quiescently accreted mass: two previously known eruptive Class I sources, V1647 Ori and EC 53 (V371 Ser), and four Class 0 sources, HOPS 356, HOPS 373, HOPS 383, and West 40. Considering the full protostellar ensemble, the importance of episodic accretion on few years timescale is negligible - only a few percent of the assembled mass. However, given that this accretion is dominated by events on the order of the observing time window, it remains uncertain as to whether the importance of episodic events will continue to rise with decades-long monitoring.

DOI： 10.3847/1538-4357/ac1679

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Scopus

Language：Japanese  

DOI： 10.1016/j.icarus.2021.114505

Open Access

Web of Science

Language：Japanese   Publisher：Astrophysical Journal  

30 Doradus C is a superbubble that emits the brightest non-thermal X- and TeV gamma-rays in the Local Group. To explore the detailed connection between the high-energy radiation and the interstellar medium, we have carried out new CO and Hi observations using the Atacama Large Millimeter/Submillimeter Array (ALMA), Atacama Submillimeter Telescope Experiment, and the Australia Telescope Compact Array with resolutions of up to 3 pc. The ALMA data of 12CO(J = 1-0) emission revealed 23 molecular clouds, with typical diameters of ∼6-12 pc and masses of ∼600-10,000 M o˙. A comparison with the X-rays of XMM-Newton at ∼3 pc resolution shows that X-rays are enhanced toward these clouds. The CO data were combined with the Hi to estimate the total interstellar protons. A comparison of the interstellar proton column density and the X-rays revealed that the X-rays are enhanced with the total proton column density. These are most likely to be caused by the shock-cloud interaction, which is modeled by magnetohydrodynamical simulations (Inoue et al. 2012). We also note a trend for the X-ray photon index to vary with distance from the center of the high-mass star cluster. This suggests that the cosmic-ray electrons are accelerated by one or multiple supernovae in the cluster. Based on these results, we discuss the role of the interstellar medium in cosmic-ray particle acceleration.

DOI： 10.3847/1538-4357/ac0adb

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Scopus

Language：Japanese   Publisher：Astrophysical Journal  

We present the results of simultaneous 450 μm and 850 μm polarization observations toward the massive star-forming region NGC 2071IR, a target of the BISTRO (B-fields in STar-forming Region Observations) Survey, using the POL-2 polarimeter and SCUBA-2 camera mounted on the James Clerk Maxwell Telescope. We find a pinched magnetic field morphology in the central dense core region, which could be due to a rotating toroidal disklike structure and a bipolar outflow originating from the central young stellar object IRS 3. Using the modified Davis–Chandrasekhar–Fermi method, we obtain a plane-of-sky magnetic field strength of 563 ± 421 μG in the central ∼0.12 pc region from 850 μm polarization data. The corresponding magnetic energy density of 2.04 × 10^-8 erg cm^-3 is comparable to the turbulent and gravitational energy densities in the region. We find that the magnetic field direction is very well aligned with the whole of the IRS 3 bipolar outflow structure. We find that the median value of polarization fractions is 3.0% at 450 μm in the central 3′ region, which is larger than the median value of 1.2% at 850 μm. The trend could be due to the better alignment of warmer dust in the strong radiation environment. We also find that polarization fractions decrease with intensity at both wavelengths, with slopes, determined by fitting a Rician noise model of 0.59 ± 0.03 at 450 μm and 0.36 ± 0.04 at 850 μm, respectively. We think that the shallow slope at 850 μm is due to grain alignment at the center being assisted by strong radiation from the central young stellar objects.

DOI： 10.3847/1538-4357/ac0ce9

Open Access

Web of Science

Scopus

Language：Japanese   Publisher：Astrophysical Journal  

Recent observations of molecular clouds show that dense filaments are the sites of present-day star formation. Thus, it is necessary to understand the filament formation process because these filaments provide the initial condition for star formation. Theoretical research suggests that shock waves in molecular clouds trigger filament formation. Since several different mechanisms have been proposed for filament formation, the formation mechanism of the observed star-forming filaments requires clarification. In the present study, we perform a series of isothermal magnetohydrodynamics simulations of filament formation. We focus on the influences of shock velocity and turbulence on the formation mechanism and identified three different mechanisms for the filament formation. The results indicate that when the shock is fast, at shock velocity v sh ≃ 7 km s-1, the gas flows driven by the curved shock wave create filaments irrespective of the presence of turbulence and self-gravity. However, at a slow shock velocity v sh ≃ 2.5 km s-1, the compressive flow component involved in the initial turbulence induces filament formation. When both the shock velocities and turbulence are low, the self-gravity in the shock-compressed sheet becomes important for filament formation. Moreover, we analyzed the line-mass distribution of the filaments and showed that strong shock waves can naturally create high-line-mass filaments such as those observed in the massive star-forming regions in a short time. We conclude that the dominant filament formation mode changes with the velocity of the shock wave triggering the filament formation.

DOI： 10.3847/1538-4357/ac07a1

Open Access

Web of Science

Scopus

Language：Japanese   Publisher：Astrophysical Journal  

We describe a numerical scheme for magnetohydrodynamics simulations of dust-gas mixture by extending smoothed particle magnetohydrodynamics. We employ the single-species particle approach to describe dust-gas mixture with several modifications from the previous studies. We assume that the charged and neutral dust can be treated as single-fluid, that the electromagnetic force acts on the gas, and that that acting on the charged dust is negligible. The validity of these assumptions in the context of protostar formation is not obvious and is extensively evaluated. By investigating the electromagnetic force and electric current with terminal velocity approximation, it is found that as the dust size increases, the contribution of dust to them becomes smaller and negligible. We conclude that our assumption that the electromagnetic force on the dusts is negligible is valid for the dust size with a d ⪆ 10 μm. On the other hand, they do not produce the numerical artifact for the dust a d ≲ 10 μm in the envelope and disk, where the perfect coupling between gas and dust is realized. However, we also found that our assumptions may break down in outflow (or under an environment with very strong magnetic field and low density) for the dust a d ≲ 10 μm. We conclude that our assumptions are valid in almost all cases where macroscopic dust dynamics is important in the context of protostar formation. We conduct numerical tests of dusty waves, dusty magnetohydrodynamics shocks, and gravitational collapse of magnetized cloud cores with our simulation code. The results show that our numerical scheme well reproduces the dust dynamics in the magnetized medium.

DOI： 10.3847/1538-4357/abf5db

Web of Science

Scopus

Language：Japanese   Publisher：Astrophysical Journal  

Optical stellar polarimetry in the Perseus molecular cloud direction is known to show a fully mixed bimodal distribution of position angles across the cloud. We study the Gaia trigonometric distances to each of these stars and reveal that the two components in position angles trace two different dust clouds along the line of sight. One component, which shows a polarization angle of -37.°6 ± 35.°2 and a higher polarization fraction of 2.0 ± 1.7 %, primarily traces the Perseus molecular cloud at a distance of 300 pc. The other component, which shows a polarization angle of +66.°8 ± 19.°1 and a lower polarization fraction of 0.8 ± 0.6 %, traces a foreground cloud at a distance of 150 pc. The foreground cloud is faint, with a maximum visual extinction of ≤1 mag. We identify that foreground cloud as the outer edge of the Taurus molecular cloud. Between the Perseus and Taurus molecular clouds, we identify a lower-density ellipsoidal dust cavity with a size of 100-160 pc. This dust cavity is located at l = 170°, b = -20°, and d = 240 pc, which corresponds to an HI shell generally associated with the Per OB2 association. The two-component polarization signature observed toward the Perseus molecular cloud can therefore be explained by a combination of the plane-of-sky orientations of the magnetic field both at the front and at the back of this dust cavity.

DOI： 10.3847/1538-4357/abfcc5

Web of Science

Scopus

Language：Japanese  

DOI： 10.1093/mnras/stab608

Open Access

Web of Science

Language：Japanese   Publisher：Astrophysical Journal Letters  

We have obtained sensitive dust continuum polarization observations at 850 μm in the B213 region of Taurus using POL-2 on SCUBA-2 at the James Clerk Maxwell Telescope as part of the B-fields in STar-forming Region Observations (BISTRO) survey. These observations allow us to probe magnetic field (B-field) at high spatial resolution (∼2000 au or ∼0.01 pc at 140 pc) in two protostellar cores (K04166 and K04169) and one prestellar core (Miz-8b) that lie within the B213 filament. Using the Davis-Chandrasekhar-Fermi method, we estimate the B-field strengths in K04166, K04169, and Miz-8b to be 38 ± 14, 44 ± 16, and 12 ± 5 μG, respectively. These cores show distinct mean B-field orientations. The B-field in K04166 is well ordered and aligned parallel to the orientations of the core minor axis, outflows, core rotation axis, and large-scale uniform B-field, in accordance with magnetically regulated star formation via ambipolar diffusion taking place in K04166. The B-field in K04169 is found to be ordered but oriented nearly perpendicular to the core minor axis and large-scale B-field and not well correlated with other axes. In contrast, Miz-8b exhibits a disordered B-field that shows no preferred alignment with the core minor axis or large-scale field. We found that only one core, K04166, retains a memory of the large-scale uniform B-field. The other two cores, K04169 and Miz-8b, are decoupled from the large-scale field. Such a complex B-field configuration could be caused by gas inflow onto the filament, even in the presence of a substantial magnetic flux.

DOI： 10.3847/2041-8213/abeb1c

Open Access

Web of Science

Scopus

Language：Japanese   Publisher：Astrophysical Journal  

In the published article, we presented high-resolution polarimetry data obtained by using JCMT SCUBA-2/POL-2, and compared them with the larger-scale magnetic structure observed by Planck (Planck Collaboration et al. 2020). There was a miscalculation in the analysis of the Planck data for comparison, and the mean position angle of the Planck magnetic field should be corrected from -40° ± 7° to -48° ± 6°. Thus, we replace the descriptions in the published article as follows. In Section 4.2, paragraph 3, the second sentence should read: "The differences between the orientations of IRAS 4A, IRAS 4B, and IRAS 2A are not statistically significant, but they do show significantly different orientations from those of the global B-field observed by Planck (-48° ± 6°; Section 4.1)." In Section 4.1, paragraph 3, the second line should read: "The Planck B-field orientation shows a smoothly and slowly varying field distribution with a position angle of -48° ± 6° in our observed NGC 1333 area." Accordingly, we replace Figures 4, 5, 9, 10, and 17 to reflect the correct Planck data. As described above, this error of modest magnitude is related only to our derivation of the Planck polarization angle, and the JCMT observation results are unaffected. Therefore, all conclusions drawn in the published article are unchanged even after the above correction is applied. The 1 pc scale magnetic field observed by Planck shows a smooth distribution, and the interstellar magnetic field in molecular clouds increases the complexity significantly on the scale of less than 1 pc.

DOI： 10.3847/1538-4357/abf2b3

Web of Science

Scopus

Language：Japanese  

DOI： 10.1093/mnras/stab608

Open Access

Web of Science

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Astronomy and Astrophysics  

Context. Molecular filaments and hubs have received special attention recently thanks to new studies showing their key role in star formation. While the (column) density and velocity structures of both filaments and hubs have been carefully studied, their magnetic field (B-field) properties have yet to be characterized. Consequently, the role of B-fields in the formation and evolution of hub-filament systems is not well constrained. Aims. We aim to understand the role of the B-field and its interplay with turbulence and gravity in the dynamical evolution of the NGC 6334 filament network that harbours cluster-forming hubs and high-mass star formation. Methods. We present new observations of the dust polarized emission at 850 μm toward the 2 pc × 10 pc map of NGC 6334 at a spatial resolution of 0.09 pc obtained with the James Clerk Maxwell Telescope (JCMT) as part of the B-field In STar-forming Region Observations (BISTRO) survey. We study the distribution and dispersion of the polarized intensity (PI), the polarization fraction (PF), and the plane-of-The-sky B-field angle (χB_POS) toward the whole region, along the 10 pc-long ridge and along the sub-filaments connected to the ridge and the hubs. We derived the power spectra of the intensity and χBPOS along the ridge crest and compared them with the results obtained from simulated filaments. Results. The observations span 3 orders of magnitude in Stokes I and PI and 2 orders of magnitude in PF (from 0.2 to 20%). A large scatter in PI and PF is observed for a given value of I. Our analyses show a complex B-field structure when observed over the whole region ( 10 pc); however, at smaller scales (1 pc), χBPOS varies coherently along the crests of the filament network. The observed power spectrum of χBPOS can be well represented with a power law function with a slope of-1.33 ± 0.23, which is 20% shallower than that of I. We find that this result is compatible with the properties of simulated filaments and may indicate the physical processes at play in the formation and evolution of star-forming filaments. Along the sub-filaments, χBPOS rotates frombeing mostly perpendicular or randomly oriented with respect to the crests to mostly parallel as the sub-filaments merge with the ridge and hubs. This variation of the B-field structure along the sub-filaments may be tracing local velocity flows of infalling matter in the ridge and hubs. Our analysis also suggests a variation in the energy balance along the crests of these sub-filaments, from magnetically critical or supercritical at their far ends to magnetically subcritical near the ridge and hubs. We also detect an increase in PF toward the high-column density (NH2 â 1023 cm-2) star cluster-forming hubs. These latter large PF values may be explained by the increase in grain alignment efficiency due to stellar radiation from the newborn stars, combined with an ordered B-field structure. Conclusions. These observational results reveal for the first time the characteristics of the small-scale (down to 0.1 pc) B-field structure of a 10 pc-long hub-filament system. Our analyses show variations in the polarization properties along the sub-filaments that may be tracing the evolution of their physical properties during their interaction with the ridge and hubs. We also detect an impact of feedback from young high-mass stars on the local B-field structure and the polarization properties, which could put constraints on possible models for dust grain alignment and provide important hints as to the interplay between the star formation activity and interstellar B-fields.

DOI： 10.1051/0004-6361/202038624

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Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Astrophysical Journal  

We observe 1.3 mm spectral lines at 2000 au resolution toward four massive molecular clouds in the Central Molecular Zone (CMZ) of the Galaxy to investigate their star formation activities. We focus on several potential shock tracers that are usually abundant in protostellar outflows, including SiO, SO, CH3OH, H2CO, HC3N, and HNCO. We identify 43 protostellar outflows, including 37 highly likely ones and 6 candidates. The outflows are found toward both known high-mass star-forming cores and less massive, seemingly quiescent cores, while 791 out of the 834 cores identified based on the continuum do not have detected outflows. The outflow masses range from less than 1 M o˙ to a few tens of M o˙, with typical uncertainties of a factor of 70. We do not find evidence of disagreement between relative molecular abundances in these outflows and in nearby analogs such as the well-studied L1157 and NGC 7538S outflows. The results suggest that (i) protostellar accretion disks driving outflows ubiquitously exist in the CMZ environment, (ii) the large fraction of candidate starless cores is expected if these clouds are at very early evolutionary phases, with a caveat on the potential incompleteness of the outflows, (iii) high-mass and low-mass star formation is ongoing simultaneously in these clouds, and (iv) current data do not show evidence of a difference between the shock chemistry in the outflows that determines the molecular abundances in the CMZ environment and in nearby clouds.

DOI： 10.3847/1538-4357/abde3c

Web of Science

Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Astrophysical Journal  

We have observed the very low-mass Class 0 protostar IRAS 15398-3359 at scales ranging from 50 to 1800 au, as part of the Atacama Large Millimeter/Submillimeter Array Large Program FAUST. We uncover a linear feature, visible in H2CO, SO, and C18O line emission, which extends from the source in a direction almost perpendicular to the known active outflow. Molecular line emission from H2CO, SO, SiO, and CH3OH further reveals an arc-like structure connected to the outer end of the linear feature and separated from the protostar, IRAS 15398-3359, by 1200 au. The arc-like structure is blueshifted with respect to the systemic velocity. A velocity gradient of 1.2 km s-1 over 1200 au along the linear feature seen in the H2CO emission connects the protostar and the arc-like structure kinematically. SO, SiO, and CH3OH are known to trace shocks, and we interpret the arc-like structure as a relic shock region produced by an outflow previously launched by IRAS 15398-3359. The velocity gradient along the linear structure can be explained as relic outflow motion. The origins of the newly observed arc-like structure and extended linear feature are discussed in relation to turbulent motions within the protostellar core and episodic accretion events during the earliest stage of protostellar evolution.

DOI： 10.3847/1538-4357/abddb1

Web of Science

Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Astrophysical Journal  

We report the first high spatial resolution measurement of magnetic fields surrounding LkHa 101, part of the Auriga- California molecular cloud. The observations were taken with the POL-2 polarimeter on the James Clerk Maxwell Telescope within the framework of the B-fields In Star-forming Region Observations (BISTRO) survey. Observed polarization of thermal dust emission at 850 μm is found to be mostly associated with the redshifted gas component of the cloud. The magnetic field displays a relatively complex morphology. Two variants of the Davis-Chandrasekhar- Fermi method, unsharp masking and structure function, are used to calculate the strength of magnetic fields in the plane of the sky, yielding a similar result of BPOS~ 115 μG. The mass-to-magnetic-flux ratio in critical value units, λ~0.3, is the smallest among the values obtained for other regions surveyed by POL-2. This implies that the LkHa 101 region is subcritical, and the magnetic field is strong enough to prevent gravitational collapse. The inferred dB/B0~0.3 implies that the large-scale component of the magnetic field dominates the turbulent one. The variation of the polarization fraction with total emission intensity can be fitted by a power law with an index of a =0.82±0.03, which lies in the range previously reported for molecular clouds. We find that the polarization fraction decreases rapidly with proximity to the only early B star (LkHa 101) in the region. Magnetic field tangling and the joint effect of grain alignment and rotational disruption by radiative torques can potentially explain such a decreasing trend.

DOI： 10.3847/1538-4357/abd0fc

Web of Science

Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Astrophysical Journal  

We compare the directions of molecular outflows of 62 low-mass Class 0 and I protostars in nearby (<450 pc) star-forming regions with the mean orientations of the magnetic fields on 0.05-0.5 pc scales in the dense cores/clumps where they are embedded. The magnetic field orientations were measured using the JCMT POL-2 data taken by the BISTRO-1 survey and from the archive. The outflow directions were observed with interferometers in the literature. The observed distribution of the angles between the outflows and the magnetic fields peaks between 15° and 35°. After considering projection effects, our results could suggest that the outflows tend to be misaligned with the magnetic fields by 50° ± 15° in three-dimensional space and are less likely (but not ruled out) randomly oriented with respect to the magnetic fields. There is no correlation between the misalignment and the bolometric temperatures in our sample. In several sources, the small-scale (1000-3000 au) magnetic field is more misaligned with the outflow than the large-scale magnetic field, suggesting that the small-scale magnetic field has been twisted by the dynamics. In comparison with turbulent MHD simulations of core formation, our observational results are more consistent with models in which the energy densities in the magnetic field and the turbulence of the gas are comparable. Our results also suggest that the misalignment alone cannot sufficiently reduce the efficiency of magnetic braking to enable formation of the observed number of large Keplerian disks with sizes larger than 30-50 au.

DOI： 10.3847/1538-4357/abca99

Web of Science

Scopus

Authorship：Lead author,　Last author,　Corresponding author   Language：Japanese   Publishing type：Research paper (scientific journal)  

Open Access

CiNii Research

Authorship：Lead author,　Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)  

Open Access

CiNii Research

Language：Japanese   Publisher：Astrophysical Journal Letters  

RX J1713.7-3946 is a unique core-collapse supernova remnant (SNR) that emits bright TeV gamma-rays and synchrotron X-rays caused by cosmic rays, in addition to interactions with interstellar gas clouds. We report here on results of Atacama Large Millimeter/submillimeter Array 12CO(J = 1-0) observations toward the northwestern shell of the SNR. We newly found three molecular complexes consisting of dozens of shocked molecular cloudlets and filaments with typical radii of ∼0.03-0.05 pc and densities of ∼104 cm-3. These cloudlets and filaments are located not only along synchrotron X-ray filaments, but also in the vicinity of X-ray hotspots with month-or year-scale time variations. We argue that X-ray hotspots and filaments were generated by shock-cloudlet interactions through magnetic-field amplification up to mG. The interstellar medium density contrast of ∼105, coexistence of molecular cloudlets and low-density diffuse medium of ∼0.1 cm-3, is consistent with such a magnetic field amplification as well as a wind-bubble scenario. The small-scale cloud structures also affect hadronic gamma-ray spectra considering the magnetic field amplification on surface and inside clouds.

DOI： 10.3847/2041-8213/abc884

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Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Astrophysical Journal  

We systematically perform hydrodynamics simulations of 20 km s-1 converging flows of the warm neutral medium (WNM) to calculate the formation of the cold neutral medium (CNM), focusing especially on the mean properties of the multiphase interstellar medium (ISM), such as the mean density on a 10 pc scale. Our results show that convergence in those mean properties requires a 0.02 pc spatial resolution that resolves the cooling length of the thermally unstable neutral medium (UNM) to follow the dynamical condensation from the WNM to the CNM. We also find that two distinct postshock states appear in the mean properties depending on the amplitude of the upstream WNM density fluctuation {equation presented}. When Δρ0 > 10 %, the interaction between shocks and density inhomogeneity leads to a strong driving of the postshock turbulence of >3 km s-1, which dominates the energy budget in the shock-compressed layer. The turbulence prevents dynamical condensation by cooling, and the CNM mass fraction remains at ∼45%. In contrast, when Δρ 0 ≥ 10%, the CNM formation proceeds efficiently, resulting in the CNM mass fraction of ∼70%. The velocity dispersion is limited to the thermal-instability-mediated level of ∼2-3 km s-1, and the layer is supported by both turbulent and thermal energy equally. We also propose an effective equation of state that models the multiphase ISM formed by the WNM converging flow as a one-phase ISM in the form of P ∝ ργeff, where γ eff varies from 0.9 (for a large pre-shock Δρ 0) to 0.7 (for a small pre-shock Δρ 0).

DOI： 10.3847/1538-4357/abc5be

Web of Science

Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： 10.1093/mnras/staa2778

Web of Science

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Astronomy and Astrophysics  

Context. Star formation takes place in giant molecular clouds, resulting in mass-segregated young stellar clusters composed of Sun-like stars, brown dwarfs, and massive O-type(50-100 M) stars. Aims. We aim to identify candidate hub-filament systems (HFSs) in the Milky Way and examine their role in the formation of the highest mass stars and star clusters. Methods. The Herschel survey HiGAL has catalogued about 105 clumps. Of these, approximately 35 000 targets are detected at the 3σ level in a minimum of four bands. Using the DisPerSE algorithm we detect filamentary skeletons on 10′ × 10′ cut-outs of the SPIRE 250 μm images (18′′ beam width) of the targets. Any filament with a total length of at least 55′′ (3 × 18′′) and at least 18′′ inside the clump was considered to form a junction at the clump. A hub is defined as a junction of three or more filaments. Column density maps were masked by the filament skeletons and averaged for HFS and non-HFS samples to compute the radial profile along the filaments into the clumps. Results. Approximately 3700 (11%) are candidate HFSs, of which about 2150 (60%) are pre-stellar and 1400 (40%) are proto-stellar. The filaments constituting the HFSs have a mean length of ∼10-20 pc, a mass of ∼5 × 104 M, and line masses (M/L) of ∼2 × 103 M pc-1. All clumps with L > 104 L and L > 105 L at distances within 2 and 5 kpc respectively are located in the hubs of HFSs. The column densities of hubs are found to be enhanced by a factor of approximately two (pre-stellar sources) up to about ten (proto-stellar sources). Conclusions. All high-mass stars preferentially form in the density-enhanced hubs of HFSs. This amplification can drive the observed longitudinal flows along filaments providing further mass accretion. Radiation pressure and feedback can escape into the inter-filamentary voids. We propose a "filaments to clusters"unified paradigm for star formation, with the following salient features: (a) low-intermediate-mass stars form slowly (106 yr) in the filaments and massive stars form quickly (105 yr) in the hub, (b) the initial mass function is the sum of stars continuously created in the HFS with all massive stars formed in the hub, (c) feedback dissipation and mass segregation arise naturally due to HFS properties, and explain the (d) age spreads within bound clusters and the formation of isolated OB associations.

DOI： 10.1051/0004-6361/202038232

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Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Astrophysical Journal  

N132D is the brightest gamma-ray supernova remnant (SNR) in the Large Magellanic Cloud (LMC). We carried out 12CO(J = 1-0, 3-2) observations toward the SNR using the Atacama Large Millimeter/submillimeter Array (ALMA) and Atacama Submillimeter Telescope Experiment. We find diffuse CO emission not only at the southern edge of the SNR as previously known, but also inside the X-ray shell. We spatially resolved nine molecular clouds using ALMA with an angular resolution of 5″, corresponding to a spatial resolution of ∼1 pc at the distance of the LMC. Typical cloud sizes and masses are ∼2.0 pc and ∼100 M o˙, respectively. High intensity ratios of CO J = 3-2/1-0 > 1.5 are seen toward the molecular clouds, indicating that shock heating has occurred. Spatially resolved X-ray spectroscopy reveals that thermal X-rays in the center of N132D are produced not only behind a molecular cloud but also in front of it. Considering the absence of a thermal component associated with the forward shock toward one molecular cloud located along the line of sight to the center of the remnant, this suggests that this particular cloud is engulfed by shock waves and is positioned on the near side of the remnant. If the hadronic process is the dominant contributor to the gamma-ray emission, the shock-engulfed clouds play a role as targets for cosmic rays. We estimate the total energy of cosmic-ray protons accelerated in N132D to be ∼0.5-3.8 × 1049 erg as a conservative lower limit, which is similar to that observed in Galactic gamma-ray SNRs.

DOI： 10.3847/1538-4357/abb469

Web of Science

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Monthly Notices of the Royal Astronomical Society  

26Al is a short-lived radioactive isotope thought to be injected into the interstellar medium (ISM) by massive stellar winds and supernovae (SNe). However, all-sky maps of 26Al emission show a distribution with a much larger scale height and faster rotation speed than either massive stars or the cold ISM. We investigate the origin of this discrepancy using an N-body + hydrodynamics simulation of a Milky-Way-like galaxy, self-consistently including self-gravity, star formation, stellar feedback, and 26Al production. We find no evidence that the Milky Way's spiral structure explains the 26Al anomaly. Stars and the 26Al bubbles they produce form along spiral arms, but, because our simulation produces material arms that arise spontaneously rather than propagating arms forced by an external potential, star formation occurs at arm centres rather than leading edges. As a result, we find a scale height and rotation speed for 26Al similar to that of the cold ISM. However, we also show that a synthetic 26Al emission map produced for a possible Solar position at the edge of a large 26Al bubble recovers many of the major qualitative features of the observed 26Al sky. This suggests that the observed anomalous 26Al distribution is the product of foreground emission from the 26Al produced by a nearby, recent SN.

DOI： 10.1093/mnras/staa2125

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Scopus

PubMed

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Astrophysical Journal  

Secular gravitational instability (GI) is one promising mechanism for creating annular substructures and planetesimals in protoplanetary disks. We perform numerical simulations of secular GI in a radially extended disk with inwardly drifting dust grains. The results show that, even in the presence of dust diffusion, dust rings form via secular GI while the dust grains are moving inward, and the dust surface density increases by a factor of 10. Once secular GI develops into a nonlinear regime, the total mass of the resultant rings can be a significant fraction of the dust disk mass. In this way, a large amount of drifting dust grains can be collected in the dusty rings and stored for planetesimal formation. In contrast to the emergence of remarkable dust substructures, secular GI does not create significant gas substructures. This result indicates that observations of a gas density profile near the disk midplane enable us to distinguish the mechanisms for creating the annular substructures in the observed disks. The resultant rings start decaying once they enter the inner region stable to secular GI. Because the ring-gap contrast smoothly decreases, it seems possible that the rings are observed even in the stable region. We also discuss the likely outcome of the nonlinear growth and indicate the possibility that a significantly developed region of secular GI may appear as a gap-like substructure in dust continuum emission as dust growth into larger solid bodies and planetesimal formation reduce the total emissivity.

DOI： 10.3847/1538-4357/abad36

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Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Astrophysical Journal Letters  

The first hydrostatic core, the first quasi-hydrostatic object formed during the star formation process, is still the observational missing link between the prestellar and protostellar phases, mainly due to its short lifetime. Although we have not established a clear method to identify this rare object, recent theoretical studies predict that the first core has millimeter continuum emission and low-velocity outflow with a wide opening angle. An extensive continuum/outflow survey toward a large number of "starless"cores in nearby star-forming regions works as a pathfinder. We observed 32 prestellar cores in Taurus with an average density of ⪆105 cm-3 in 1.3 mm continuum and molecular lines using the Atacama Large Millimeter/submillimeter Array-Atacama Compact Array (ALMA-ACA) stand-alone mode. Among the targets, MC35-mm centered at one of the densest "starless"cores in Taurus has blueshifted/redshifted wings in the 12CO (2-1) line, indicating that there is a deeply embedded object driving molecular outflow. The observed velocities and sizes of the possible outflow lobes are 2-4 km s-1 and ∼2 × 103 au, respectively, and the dynamical time is calculated to be ∼103 yr. In addition to this, the core is one of the strongest N2D+ (3-2) emitters in our sample. All of the observed signatures do not conflict with any of the theoretical predictions about the first hydrostatic core so far, and thus MC35-mm is unique as the only first-core candidate in the Taurus molecular cloud.

DOI： 10.3847/2041-8213/ab9ca8

Web of Science

Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Astrophysical Journal  

We present new observations of the active star formation region NGC 1333 in the Perseus molecular cloud complex from the James Clerk Maxwell Telescope B-Fields In Star-forming Region Observations (BISTRO) survey with the POL-2 instrument. The BISTRO data cover the entire NGC 1333 complex (∼1.5 pc ? 2 pc) at 0.02 pc resolution and spatially resolve the polarized emission from individual filamentary structures for the first time. The inferred magnetic field structure is complex as a whole, with each individual filament aligned at different position angles relative to the local field orientation. We combine the BISTRO data with low- and high- resolution data derived from Planck and interferometers to study the multiscale magnetic field structure in this region. The magnetic field morphology drastically changes below a scale of ∼1 pc and remains continuous from the scales of filaments (∼0.1 pc) to that of protostellar envelopes (∼0.005 pc or ∼1000 au). Finally, we construct simple models in which we assume that the magnetic field is always perpendicular to the long axis of the filaments. We demonstrate that the observed variation of the relative orientation between the filament axes and the magnetic field angles are well reproduced by this model, taking into account the projection effects of the magnetic field and filaments relative to the plane of the sky. These projection effects may explain the apparent complexity of the magnetic field structure observed at the resolution of BISTRO data toward the filament network.

DOI： 10.3847/1538-4357/aba1e2

Web of Science

Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Astrophysical Journal  

We have performed survey-type observations in 1 mm continuum and molecular lines toward dense cores (32 prestellar + 7 protostellar) with an average density of ⪆105 cm-3 in the Taurus molecular clouds using the Atacama Large Millimeter/submillimeter Array-Atacama Compact Array (ALMA-ACA) stand-alone mode with an angular resolution of 6.″5 (∼900 au). The primary purpose of this study is to investigate the innermost part of dense cores with view to understanding the initial condition of star formation. In the protostellar cores, contributions from protostellar disks dominate the observed continuum flux with a range of 35%-90%, except for the very low-luminosity object. For the prestellar cores, we have successfully confirmed continuum emission from dense gas with a density of ⪆3 × 105 cm-3 toward approximately one-third of the targets. Thanks to the lower spatial frequency coverage with the ACA 7 m array, the detection rate is significantly higher than that of the previous surveys, which have zero or one continuum-detected sources among a large number of starless samples using the ALMA Main Array. The statistical counting method tells us that the lifetime of prestellar cores until protostar formation therein approaches the freefall time as the density increases. Among the prestellar cores, at least two targets have possible internal substructures, which are detected in continuum emission with the size scale of ∼1000 au if we consider the molecular line (C18O and N2D+) distributions. These results suggest that small-scale fragmentation/coalescence processes occur in a region smaller than 0.1 pc, which may determine the final core mass associated with individual protostar formation before starting the dynamical collapse of the core with a central density of ∼(0.3-1) × 106 cm-3.

DOI： 10.3847/1538-4357/ab9ca7

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Scopus

Language：Japanese   Publishing type：Research paper (international conference proceedings)   Publisher：Space Science Reviews  

In this chapter, we review some historical understanding and recent advances on the Initial Mass Function (IMF) and the Core Mass Function (CMF), both in terms of observations and theories. We focus mostly on star formation in clustered environment since this is suggested by observations to be the dominant mode of star formation. The statistical properties and the fragmentation behaviour of turbulent gas is discussed, and we also discuss the formation of binaries and small multiple systems.

DOI： 10.1007/s11214-020-00699-2

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Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Astrophysical Journal  

The formation and early evolution of low-mass young stellar objects (YSOs) are investigated using three-dimensional non-ideal magnetohydrodynamics simulations. We investigate the evolution of YSOs up to ∼ 104 after protostar formation, at which protostellar mass reaches ∼ 0.1M⊙. We particularly focus on the impact of the dust model on the evolution. We found that a circumstellar disk is formed in all simulations, regardless of the dust model. Disk size is approximately 10 au at the protostar formation epoch, and it increases to several tens of au at ∼ 104 after protostar formation. The disk mass is comparable to the central protostellar mass, and gravitational instability develops. In simulations with small dust sizes, the warp of the pseudodisk develops ∼ 104 after protostar formation. The warp strengthens magnetic braking in the disk and decreases disk size. Ion-neutral drift can occur in the infalling envelope when the typical dust size is ≳ 0.2μ m and the protostar (plus disk) mass is M≳ 0.1M⊙. The outflow activity is anticorrelated to the dust size, and the strong outflow appears with small dust grains.

DOI： 10.3847/1538-4357/ab93d0

Open Access

Web of Science

Scopus

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：Space Science Reviews  

Giant molecular clouds (GMCs) and their stellar offspring are the building blocks of galaxies. The physical characteristics of GMCs and their evolution are tightly connected to galaxy evolution. The macroscopic properties of the interstellar medium propagate into the properties of GMCs condensing out of it, with correlations between e.g. the galactic and GMC scale gas pressures, surface densities and volume densities. That way, the galactic environment sets the initial conditions for star formation within GMCs. After the onset of massive star formation, stellar feedback from e.g. photoionisation, stellar winds, and supernovae eventually contributes to dispersing the parent cloud, depositing energy, momentum and metals into the surrounding medium, thereby changing the properties of galaxies. This cycling of matter between gas and stars, governed by star formation and feedback, is therefore a major driver of galaxy evolution. Much of the recent debate has focused on the durations of the various evolutionary phases that constitute this cycle in galaxies, and what these can teach us about the physical mechanisms driving the cycle. We review results from observational, theoretical, and numerical work to build a dynamical picture of the evolutionary lifecycle of GMC evolution, star formation, and feedback in galaxies.

DOI： 10.1007/s11214-020-00674-x

Open Access

Web of Science

Scopus

PubMed

Authorship：Lead author,　Last author,　Corresponding author   Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Japanese Society for Planetary Sciences  

DOI： 10.14909/yuseijin.29.1_3

Open Access

CiNii Research

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Monthly Notices of the Royal Astronomical Society  

We investigate the roles of magnetically driven disc wind (MDW) and thermally driven photoevaporative wind (PEW) in the long-time evolution of protoplanetary discs. We start simulations from the early phase in which the disc mass is 0.118M⊙ around a 1M⊙ star and track the evolution until the disc is completely dispersed. We incorporate the mass-loss by PEW and the mass-loss and magnetic braking (wind torque) by MDW, in addition to the viscous accretion, viscous heating, and stellar irradiation. We find that MDW and PEW, respectively, have different roles: Magnetically driven wind ejects materials from an inner disc in the early phase, whereas photoevaporation has a dominant role in the late phase in the outer (≳1 au) disc. The disc lifetime, which depends on the combination of MDW, PEW, and viscous accretion, shows a large variation of ∼1-20 Myr; the gas is dispersed mainly by the MDW and the PEW in the cases with a low viscosity and the lifetime is sensitive to the mass-loss rate and torque of the MDW, whereas the lifetime is insensitive to these parameters when the viscosity is high. Even in discs with very weak turbulence, the cooperation of MDW and PEW enables the disc dispersal within a few Myr.

DOI： 10.1093/mnras/staa087

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Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Planetary and Space Science  

The members of asteroid families have various shapes. We investigate the origin of their shapes by high-resolution impact simulations for catastrophic disruptions using a Smoothed Particle Hydrodynamics code. Collisional remnants produced through our simulations of the catastrophic disruptions mainly have spherical or bilobed shapes. However, no flat remnants with the ratio of minor to major axis lengths c/a≲0.5 are formed. The results of the simulations provide various shapes of asteroids and explain most of the shapes in asteroid families that are supposed to be produced through catastrophic disruptions. However, the present simulations do not explain significantly flat asteroids. We suggest that these flat asteroids may be interlopers or formed through low-velocity collisions between member asteroids.

DOI： 10.1016/j.pss.2019.104807

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Scopus

Authorship：Last author   Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Japanese Society for Planetary Sciences  

DOI： 10.14909/yuseijin.28.4_323

Open Access

CiNii Research

Language：Japanese   Publisher：Publications of the Astronomical Society of Japan  

Recent observations of the nearby Galactic molecular clouds indicate that the dense gas in molecular clouds has quasi-universal properties on star formation, and observational studies of extra-galaxies have shown a galactic-scale correlation between the star formation rate (SFR) and the surface density of molecular gas. To reach a comprehensive understanding of both properties, it is important to quantify the fractional mass of dense gas in molecular clouds, f<inf>DG</inf>. In particular, for the Milky Way (MW) there are no previous studies resolving f<inf>DG</inf> disk over a scale of several kpc. In this study, f<inf>DG</inf> was measured over 5 kpc in the first quadrant of the MW, based on the CO J = 1-0 data in l = 10°-50° obtained as part of the FOREST Unbiased Galactic plane Imaging survey with the Nobeyama 45 m telescope (FUGIN) project. The total molecular mass was measured using ¹²CO, and the dense gas mass was estimated using C¹⁸O. The fractional masses, including f<inf>DG</inf>, in the region within ±30% of the distances to the tangential points of the Galactic rotation (e.g., the Galactic Bar, Far-3 kpc Arm, Norma Arm, Scutum Arm, Sagittarius Arm, and inter-arm regions) were measured. As a result, an averaged f<inf>DG</inf> of $2.9^{+2.6}_{-2.6}$% was obtained for the entirety of the target region. This low value suggests that dense gas formation is the primary factor in inefficient star formation in galaxies. It was also found that f<inf>DG</inf> shows large variations depending on the structures in the MW disk. In the Galactic arms, f<inf>DG</inf> was estimated to be ∼4%-5%, while in the bar and inter-arm regions it was as small as ∼0.1%-0.4%. These results indicate that the formation/destruction processes of the dense gas and their timescales are different for different regions in the MW, leading to differences in Star formation efficiencies.

DOI： 10.1093/pasj/psz033

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Scopus

Authorship：Last author   Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Japanese Society for Planetary Sciences  

DOI： 10.14909/yuseijin.28.3_172

Open Access

CiNii Research

Language：Japanese   Publisher：Monthly Notices of the Royal Astronomical Society  

We perform radiation hydrodynamical simulations in spherical symmetry in order to investigate the formation of very low mass objects, i.e. brown dwarfs, by external compression. According to the Jeans stability criterion, a very low mass molecular cloud core must reach a very high density in order to become gravitationally unstable. One possibility to create such a high density is the compression by turbulent flows within the larger molecular cloud. Using our self-developed radiation hydrodynamics code, we aim to test the validity of this scenario, and to constrain the strength of the turbulence that is needed. We find that the probability for sufficiently strong and long-lived turbulence is very low under typical conditions even when using very optimistic assumptions, and therefore conclude that turbulent compression is unlikely to be the dominant mechanism for creating brown dwarfs. We also investigate the properties of objects formed by this turbulent compression process. Specifically, we compare the lifetime of the first core stage for the cases with and without external compression. We confirm our previous findings that the first core lifetime increases by about an order of magnitude at the extremely low-mass end, but this increase is somewhat less dramatic and occurs at even lower masses than in our previous work, in which no external compression was present.

DOI： 10.1093/mnras/stz1892

Open Access

Web of Science

Scopus

Language：Japanese   Publisher：Astrophysical Journal  

The angular momentum of a molecular cloud core plays a key role in star formation, as it is directly related to the outflow and the jet emanating from the newborn star, and it eventually results in the formation of the protoplanetary disk. However, the origin of the core rotation and its time evolution are not well understood. Recent observations reveal that molecular clouds exhibit a ubiquity of filamentary structures and that star-forming cores are associated with the densest filaments. As these results suggest that dense cores form primarily in filaments, the mechanism of core formation from filament fragmentation should explain the distribution of the angular momentum of these cores. In this paper we analyze the relation between velocity fluctuations along the filament close to equilibrium, and the angular momentum of the cores formed along its crest. We first find that an isotropic velocity fluctuation that follows the three-dimensional Kolmogorov spectrum does not reproduce the observed angular momentum of molecular cloud cores. We then identify the need for a large power at small scales and study the effect of three power spectrum models. We show that the one-dimensional Kolmogorov power spectrum with a slope of -5/3 and an anisotropic model with reasonable parameters are compatible with the observations. Our results stress the importance of more detailed and systematic observations of both the velocity structure along filaments, and the angular momentum distribution of molecular cloud cores, to determine the validity of the mechanism of core formation from filamentary molecular clouds.

DOI： 10.3847/1538-4357/ab2382

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Scopus

Language：Japanese   Publisher：Astrophysical Journal  

Various instabilities have been proposed as a promising mechanism for accumulating dust. Moreover, some of them are expected to lead to multiple-ring structure formation and planetesimal formation in protoplanetary disks. In a turbulent gaseous disk, the growth of the instabilities and the dust accumulation are quenched by the turbulent diffusion of dust grains. The diffusion process has often been modeled by a diffusion term in the continuity equation for the dust density. The dust diffusion model, however, does not guarantee conservation of angular momentum in a disk. In this study, we first formulate equations that describe dust diffusion and also conserve the total angular momentum of a disk. Second, we perform a linear perturbation analysis on the secular gravitational instability (GI) using the equations. The results show that the secular GI is a monotonically growing mode, contrary to the result of previous analyses that found it overstable. We find that the overstability is caused by the nonconservation of the angular momentum. Third, we find a new axisymmetric instability due to the combination of dust-gas friction and turbulent gas viscosity, which we refer to as two-component viscous gravitational instability (TVGI). The most unstable wavelength of TVGI is comparable to or smaller than the gas scale height. TVGI accumulates dust grains efficiently, which indicates that TVGI is a promising mechanism for the formation of multiple-ring-like structures and planetesimals. Finally, we examine the validity of the ring formation via the secular GI and TVGI in the HL Tau disk and find both instabilities can create multiple rings whose width is about 10 au at orbital radii larger than 50 au.

DOI： 10.3847/1538-4357/ab25ea

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Scopus

Language：Japanese   Publisher：Astrophysical Journal  

RX J0046.5-7308 is a shell-type supernova remnant (SNR) in the Small Magellanic Cloud (SMC). We carried out new ¹²CO(J = 1-0, 3-2) observations toward the SNR using Mopra and the Atacama Submillimeter Telescope Experiment. We found eight molecular clouds (A-H) along the X-ray shell of the SNR. The typical cloud size and mass are ∼10-15 pc and ∼1000-3000 M , respectively. The X-ray shell is slightly deformed and has the brightest peak in the southwestern shell where two molecular clouds A and B are located. The four molecular clouds A, B, F, and G have high intensity ratios of ¹²CO(J = 3-2)/¹²CO(J = 1-0) > 1.2, which are not attributable to any identified internal infrared sources or high-mass stars. The H i cavity and its expanding motion are found toward the SNR, which are likely created by strong stellar winds from a massive progenitor. We suggest that the molecular clouds A-D, F, and G and H i clouds within the wind-blown cavity at V <inf>LSR</inf> = 117.1-122.5 km s^-1 are associated with the SNR. The X-ray spectroscopy reveals the dynamical age of yr and the progenitor mass of ≳30 M , which is also consistent with the proposed scenario. We determine physical conditions of the giant molecular cloud LIRS 36A using the large velocity gradient analysis with archival data sets of the Atacama Large Millimeter/submillimeter Array; the kinematic temperature is K and the number density of molecular hydrogen is cm^-3. The next generation of γ-ray observations will allow us to study the pion-decay γ-rays from the molecular clouds in the SMC SNR.

DOI： 10.3847/1538-4357/ab2ade

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Scopus

Language：Japanese   Publisher：Publications of the Astronomical Society of Japan  

The formation scenario of brown dwarfs is still unclear because observational studies to investigate its initial condition are quite limited. Our systematic survey of nearby low-mass star-forming regions using the Atacama Compact Array (aka the Morita array) and the IRAM 30-m telescope in 1.2 mm continuum has identified a centrally concentrated starless condensation with a central H<inf>2</inf> volume density of ∼10⁶ cm^-3, MC5-N, connected to a narrow (width ∼0.03 pc) filamentary cloud in the Taurus L1495 region. The mass of the core is ∼ 0.2-0.4, Mo , which is an order of magnitude smaller than typical low-mass pre-stellar cores. Taking into account a typical core to star formation efficiency for pre-stellar cores (∼20%-40%) in nearby molecular clouds, brown dwarf(s) or very low-mass star(s) may be going to be formed in this core. We have found possible substructures at the high-density portion of the core, although much higher angular resolution observation is needed to clearly confirm them. The subsequent N<inf>2</inf>H⁺ and N<inf>2</inf>D⁺ observations using the Nobeyama 45-m telescope have confirmed the high-deuterium fractionation (∼30%). These dynamically and chemically evolved features indicate that this core is on the verge of proto-brown dwarf or very low-mass star formation and is an ideal source to investigate the initial conditions of such low-mass objects via gravitational collapse and/or fragmentation of the filamentary cloud complex.

DOI： 10.1093/pasj/psz051

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Scopus

Language：Japanese   Publisher：Icarus  

Light curve observations of a recently discovered interstellar object 1I/‘Oumuamua suggest that this object has an extremely elongated shape with the axis ratio 0.3 or smaller. Planetesimal collisions can produce irregular shapes including elongated shapes. In this paper, we suggest that the extremely elongated shape of 1I/‘Oumuamua may be the result of such an impact. To find detailed impact conditions to form the extremely elongated objects, we conduct numerical simulations of planetesimal collisions using Smoothed Particle Hydrodynamics method for elastic dynamics with self-gravity and interparticle friction. Impacts into strengthless target planetesimals with radius 50 m are conducted with various ratios of impactor mass to target mass q, friction angles ϕ <inf>d</inf> , impact velocities v <inf>imp</inf> , and impact angles θ <inf>imp</inf> . We find that impacts with q ≥ 0.5, ϕ <inf>d</inf> ≥ 40 ^° , v <inf>imp</inf> ≤ 40 cm/s, and θ <inf>imp</inf> ≤ 30 ^° produce remnants with the ratio of intermediate to major axis length <0.3. This impact condition suggests that the parent protoplanetary disk in the planetesimal collision stage was weakly turbulent (α < 10 ⁻⁴ for the inner disk) and composed of planetesimals smaller than ∼7 km to ensure small impact velocity.

DOI： 10.1016/j.icarus.2019.03.014

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Scopus

Language：Japanese   Publisher：Astrophysical Journal  

The magnetohydrodynamics (MHD) of protoplanetary disks are strongly subject to the nonideal MHD effects arising from the low ionization fraction of the disk gas. A strong electric field induced by gas motions can heat ionized gas particles and can thereby affect the ionization balance in the disks. Our previous studies revealed that in dusty protoplanetary disks, the ohmic conductivity decreases with increasing electric field strength until the electrical breakdown of the disk gas occurs. In this study, we extend our previous work to more general cases where both electric and magnetic fields affect the motion of plasma particles, allowing us to study the impacts of plasma heating on all nonideal MHD effects: ohmic, Hall, and ambipolar diffusion. We find that the upper limit on the electric current we previously derived applies even in the presence of magnetic fields. Although the Hall and ambipolar resistivities can either increase or decrease with electric field strength depending on the abundance of charged dust grains, the ohmic resistivity always increases with electric field strength. An order-of-magnitude estimate suggests that a large-scale electric current generated by gas motions in the inner part of protoplanetary disks could exceed the upper limit. This implies that MHD motions of the inner disk, such as the motion driven by the Hall-shear instability, could either get suppressed or trigger electrical breakdown (lightning discharge). This may have important implications for gas accretion and chondrule formation in the inner part of protoplanetary disks.

DOI： 10.3847/1538-4357/ab2046

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Scopus

Language：Japanese   Publisher：Astrophysical Journal  

We present the POL-2 850 μm linear polarization map of the Barnard 1 clump in the Perseus molecular cloud complex from the B-fields In STar-forming Region Observations survey at the James Clerk Maxwell Telescope. We find a trend of decreasing polarization fraction as a function of total intensity, which we link to depolarization effects toward higher-density regions of the cloud. We then use the polarization data at 850 μm to infer the plane-of-sky orientation of the large-scale magnetic field in Barnard 1. This magnetic field runs north-south across most of the cloud, with the exception of B1-c, where it turns more east-west. From the dispersion of polarization angles, we calculate a turbulence correlation length of 5.0 ±2.″5 (1500 au) and a turbulent-to-total magnetic energy ratio of 0.5 ±0.3 inside the cloud. We combine this turbulent-to-total magnetic energy ratio with observations of NH<inf>3</inf> molecular lines from the Green Bank Ammonia Survey to estimate the strength of the plane-of-sky component of the magnetic field through the Davis-Chandrasekhar-Fermi method. With a plane-of-sky amplitude of 120 ±60 μG and a criticality criterion λ <inf>c</inf> = 3.0 ±1.5, we find that Barnard 1 is a supercritical molecular cloud with a magnetic field nearly dominated by its turbulent component.

DOI： 10.3847/1538-4357/ab1b23

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Scopus

Language：Japanese   Publisher：Astrophysical Journal  

We report 850 μm dust polarization observations of a low-mass (∼12 M <inf>o</inf>) starless core in the ρ Ophiuchus cloud, Ophiuchus C, made with the POL-2 instrument on the James Clerk Maxwell Telescope (JCMT) as part of the JCMT B-fields In STar-forming Region Observations survey. We detect an ordered magnetic field projected on the plane of the sky in the starless core. The magnetic field across the ∼0.1 pc core shows a predominant northeast-southwest orientation centering between ∼40° and ∼100°, indicating that the field in the core is well aligned with the magnetic field in lower-density regions of the cloud probed by near-infrared observations and also the cloud-scale magnetic field traced by Planck observations. The polarization percentage (P) decreases with increasing total intensity (I), with a power-law index of -1.03 ± 0.05. We estimate the plane-of-sky field strength (B <inf>pos</inf>) using modified Davis-Chandrasekhar-Fermi methods based on structure function (SF), autocorrelation function (ACF), and unsharp masking (UM) analyses. We find that the estimates from the SF, ACF, and UM methods yield strengths of 103 ± 46 μG, 136 ± 69 μG, and 213 ± 115 μG, respectively. Our calculations suggest that the Ophiuchus C core is near magnetically critical or slightly magnetically supercritical (i.e., unstable to collapse). The total magnetic energy calculated from the SF method is comparable to the turbulent energy in Ophiuchus C, while the ACF method and the UM method only set upper limits for the total magnetic energy because of large uncertainties.

DOI： 10.3847/1538-4357/ab0958

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Scopus

Language：Japanese   Publisher：Astrophysical Journal  

We present the 850 μm polarization observations toward the IC 5146 filamentary cloud taken using the Submillimetre Common-User Bolometer Array 2 (SCUBA-2) and its associated polarimeter (POL-2), mounted on the James Clerk Maxwell Telescope, as part of the B-fields In STar forming Regions Observations. This work is aimed at revealing the magnetic field morphology within a core-scale (≲1.0 pc) hub-filament structure (HFS) located at the end of a parsec-scale filament. To investigate whether the observed polarization traces the magnetic field in the HFS, we analyze the dependence between the observed polarization fraction and total intensity using a Bayesian approach with the polarization fraction described by the Rice likelihood function, which can correctly describe the probability density function of the observed polarization fraction for low signal-to-noise ratio data. We find a power-law dependence between the polarization fraction and total intensity with an index of 0.56 in A <inf>V</inf> ∼ 20-300 mag regions, suggesting that the dust grains in these dense regions can still be aligned with magnetic fields in the IC 5146 regions. Our polarization maps reveal a curved magnetic field, possibly dragged by the contraction along the parsec-scale filament. We further obtain a magnetic field strength of 0.5 ±; 0.2 mG toward the central hub using the Davis-Chandrasekhar-Fermi method, corresponding to a mass-to-flux criticality of ∼1.3 ±; 0.4 and an Alfvénic Mach number of <0.6. These results suggest that gravity and magnetic field are currently of comparable importance in the HFS and that turbulence is less important.

DOI： 10.3847/1538-4357/ab13a2

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Scopus

Language：Japanese   Publisher：Astrophysical Journal  

We present a detailed morphological study of TeV gamma-rays, synchrotron radiation, and interstellar gas in the young Type Ia supernova remnant (SNR) RCW 86. We find that the interstellar atomic gas shows good spatial correlation with the gamma-rays, indicating that the TeV gamma-rays from RCW 86 are likely predominantly of hadronic origin. In contrast, the spatial correlation between the interstellar molecular cloud and the TeV gamma-rays is poor in the southeastern shell of the SNR. We argue that this poor correlation can be attributed to the low-energy cosmic rays (∼1 TeV) not penetrating into the dense molecular cloud due to an enhancement of the turbulent magnetic field around the dense cloud of ∼10-100 μG. We also find that the southwestern shell, which is bright in both synchrotron X-ray and radio continuum radiation, shows a significant gamma-ray excess compared with the interstellar proton column density, suggesting that leptonic gamma-rays via inverse Compton scattering possibly contribute alongside the hadronic gamma-rays. The total cosmic-ray energies of the young TeV gamma-ray SNRs - RX J1713.7-3946, Vela Jr, HESS J1731-347, and RCW 86 - are roughly similar, which indicates that cosmic rays can be accelerated in both the core-collapse and Type Ia supernovae. The total energy of cosmic rays derived using the gas density, ∼10⁴⁸-10⁴⁹ erg, gives a safe lower limit due mainly to the low filling factor of interstellar gas within the shell.

DOI： 10.3847/1538-4357/ab108f

Open Access

Web of Science

Scopus

Publisher：Frontiers in Astronomy and Space Sciences  

We review the role that magnetic field may have on the formation and evolution of molecular clouds. After a brief presentation and main assumptions leading to ideal MHD equations, their most important correction, namely the ion-neutral drift is described. The nature of the multi-phase interstellar medium (ISM) and the thermal processes that allows this gas to become denser are presented. Then we discuss our current knowledge of compressible magnetized turbulence, thought to play a fundamental role in the ISM. We also describe what is known regarding the correlation between the magnetic and the density fields. Then the influence that magnetic field may have on the interstellar filaments and the molecular clouds is discussed, notably the role it may have on the pre-stellar dense cores as well as regarding the formation of stellar clusters. Finally we briefly review its possible effects on the formation of molecular clouds themselves. We argue that given the magnetic intensities that have been measured, it is likely that magnetic field is (i) responsible of reducing the star formation rate in dense molecular cloud gas by a factor of a few, (ii) strongly shaping the interstellar gas by generating a lot of filaments and reducing the numbers of clumps, cores and stars, although its exact influence remains to be better understood. Moreover at small scales, magnetic braking is likely a dominant process that strongly modifies the outcome of the star formation process. Finally, we stress that by inducing the formation of more massive stars, magnetic field could possibly enhance the impact of stellar feedback.

DOI： 10.3389/fspas.2019.00005

Open Access

Web of Science

Scopus

Publisher：Astrophysical Journal  

We carried out new ¹² CO(J = 1-0, 3-2) observations of a N63A supernova remnant (SNR) from the LMC using the Atacama Large Millimeter/submillimeter Array (ALMA) and Atacama Submillimeter Telescope Experiment. We find three giant molecular clouds toward the northeast, east, and near the center of the SNR. Using the ALMA data, we spatially resolved clumpy molecular clouds embedded within the optical nebulae in both the shock-ionized and photoionized lobes discovered by previous H and [S ii] observations. The total mass of the molecular clouds is ∼800 M for the shock-ionized region and ∼1700 M for the photoionized region. Spatially resolved X-ray spectroscopy reveals that the absorbing column densities toward the molecular clouds are ∼(1.5-6.0) ? 10 ²¹ cm ^-2 , which are ∼1.5-15 times less than the averaged interstellar proton column densities for each region. This means that the X-rays are produced not only behind the molecular clouds, but also in front of them. We conclude that the dense molecular clouds have been completely engulfed by the shock waves, but have still survived erosion owing to their high density and short interacting time. The X-ray spectrum toward the gas clumps is well explained by an absorbed power-law model or a high-temperature plasma model, in addition to thermal plasma components, implying that the shock-cloud interaction is efficiently working for both cases through the shock ionization and magnetic field amplification. If the hadronic gamma-ray is dominant in the GeV band, the total energy of the cosmic-ray protons is calculated to be ∼(0.3-1.4) ? 10 ⁴⁹ erg, with an estimated interstellar proton density of ∼190 ? 90 cm ^-3 , containing both the shock-ionized gas and neutral atomic hydrogen.

DOI： 10.3847/1538-4357/ab02fd

Open Access

Web of Science

Scopus

Publisher：Astrophysical Journal  

We investigate the formation of molecular clouds from atomic gas by using three-dimensional magnetohydrodynamic simulations, including non-equilibrium chemical reactions and heating/cooling processes. We consider super-Alfvénic head-on colliding flows of atomic gas possessing the two-phase structure that consists of H i clouds and surrounding warm diffuse gas. We examine how the formation of molecular clouds depends on the angle θ between the upstream flow and the mean magnetic field. We find that there is a critical angle θ <inf>cr</inf> above which the shock-amplified magnetic field controls the post-shock gas dynamics. If the atomic gas is compressed almost along the mean magnetic field (θ ≪ θ <inf>cr</inf> ), super-Alfvénic anisotropic turbulence is maintained by the accretion of the highly inhomogeneous upstream atomic gas. As a result, a greatly extended turbulence-dominated post-shock layer is generated. Around θ ∼ θ <inf>cr</inf> , the shock-amplified magnetic field weakens the post-shock turbulence, leading to a dense post-shock layer. For θ ≫ θ <inf>cr</inf> , the strong magnetic pressure suppresses the formation of cold dense clouds. Efficient molecular cloud formation is expected if θ is less than a few times θ <inf>cr</inf> . Developing an analytic model and performing a parameter survey, we obtain an analytic formula for the critical angle as a function of the mean density, collision speed, and field strength of the upstream atomic gas. The critical angle is found to be less than ∼15° as long as the field strength is larger than 1 μG, indicating that the probability of occurrence of compression with θ < θ <inf>cr</inf> is limited if shock waves come from various directions.

DOI： 10.3847/1538-4357/ab02ff

Web of Science

Scopus

Publisher：Astronomical Journal  

In our paper (Takahashi & Inutsuka 2016), there are typographical errors in Equations (13) and (17). The correct equations are σφ = -2πG/|κ| (σ∑/1 + kH + σ∑ <inf>d</inf>/1 + kH<inf>d</inf>), c<inf>d</inf>² = 1 + 2(t<inf>stop</inf>ω)² + (5/4) (t<inf>stop</inf>ω)³/[1 + (t<inf>stop</inf>ω)²]² αc<inf>s</inf>². respectively. Since we used the correct equations in our calculations, these typographical errors do not change the dispersion relation in our original paper. Apart from the above typographical errors, we used an incorrect equation for the dust diffusion by the turbulence to calculate the dispersion relations. We underestimated (overestimated) the diffusion of dust particles with small (large) Stoke number. The secular GI is stabilized when we use the corrected equation with the exponential cutoff surface density model with the parameters given in the original paper, since the secular GI was marginally unstable with the parameters used in the original paper. Thus, we show the alternative results whose strength of the turbulence α are 2/3 times smaller than that used in the original paper. The fiducial value of α in the original paper is observationally suggested by Pinte et al. (2016). Since this α value is required from the observed sharp ring structure, this value should be regarded as an upper limit. Thus, the observation of the continuum emission of the disk around HL Tau can be explained by α = 2 × 10^-4, with which the secular GI grows. Figures 1-5 are the same as Figures 1-5 in the original paper, but here we use α = 2 × 10^-4 instead of the value used in the original paper, α = 3 × 10^-4. In Figure 3, the growth timescale in the upper right region is of the order of 10<inf>4</inf> yr, in which the Secular GI is stable in the original paper. In this region, the Stokes number becomes larger than unity. In the disk around HL Tau, however, it does not seem realized. We do not discuss the details of the instability in this region, which is beyond the scope of the paper. For Figures 6 and 7, we use α1 = 2 × 10^-4(r/100 au)², α2 = 2 × 10^-4(r/100 au)³, instead of Equations (22) and (23) in the original paper. Since the growth timescales and the wavenumber in Figures 1-7 are similar to that in the original paper, all the conclusions in the original paper remain unchanged. We would like to thank Nicolai Skovby for pointing out our typographical errors and reproducing the dispersion relation, which drew our attention to our error. We also thank Ryosuke Tominaga for numerical comparison of the results in this erratum.

DOI： 10.3847/1538-3881/ab01d0

Web of Science

Scopus

Publisher：Astrophysical Journal  

In our published article (Takahashi & Inutsuka 2014), there are typographical errors in Equations (26) and (29). The correct equations are (Formula Presented) respectively. Since we used the correct equations in our calculations, these typographical errors do not change the dispersion relation in our published article. We also found that the dispersion relation in Figure 2 is incorrect. The corrected Figure 2 is provided here. We used Equations (6), (9)-(12), (23), and (24) for the solid line in Figure 2, which means that we did not take into account the effect of the dust velocity dispersion and the disk thickness. When we use Equations (6), (9), (11), (23), (28), and (29) in the published article, the secular gravitational instability (GI) is stabilized with the parameters used in Figure 2. Therefore, we provide the alternative figure here (Figure 2). Figure 2 shows the dispersion relation with t <inf>stop</inf> ω = 0.02, Q = 3, ϵ = 0.1, and α = 10 ^-4 . The solid line shows the dispersion relation calculated from Equations (6), (9), (11), (23), (28), and (29); and dashed line shows that calculated from Equations (6)-(12). Figure 2 shows that the growth rate of the instability decreases due to the turbulent viscosity, the dust velocity dispersion, and the disk thickness. All the discussions and conclusions in the published article remain unchanged. We would like to thank Nicolai Skovby for pointing out our typographical errors and reproducing the dispersion relation, which drew our attention to our error. We also thank Ryosuke Tominaga for numerical comparison of the results in this erratum. (Figure Presented).

DOI： 10.3847/1538-4357/aafc28

Web of Science

Scopus

Publisher：Astronomical Journal  

Uranus has a tilted rotation axis, which is supposed to have been caused by a giant impact. In general, an impact event also changes the internal compositional distribution and drives mass ejection from the planet, which may provide the origin of satellites. Previous studies of the impact simulation of Uranus investigated the resultant angular momentum and the ejected mass distribution. However, the effect of changing the initial condition of the thermal and compositional structure is not studied. In this paper, we perform hydrodynamics simulations for the impact events of Uranus-size ice giants composed of a water core surrounded by a hydrogen envelope using two variant methods of the smoothed particle hydrodynamics. We find that the higher-entropy target loses its envelope more efficiently than the low-entropy target. However, the higher-entropy target gains more angular momentum than the lower-entropy target since the higher-entropy target has a more expanded envelope. We discuss the efficiency of angular momentum transport and the amount of the ejected mass and find a simple analytical model to roughly reproduce the outcomes of numerical simulations. We suggest the range of possible initial conditions for the giant impact on proto-Uranus that reproduces the present rotation tilt of Uranus and sufficiently provides the total angular momentum of the satellite system that can be created from the fragments from the giant impact.

DOI： 10.3847/1538-3881/aaf165

Web of Science

Scopus

Publisher：Astrophysical Journal  

We perform radiation-hydrodynamical simulations of protostellar collapse in spherical symmetry, with a special focus on very low-mass objects, i.e., brown dwarfs and sub-brown dwarfs. The inclusion of a realistic equation of state, which includes the effect of hydrogen dissociation, allows for a modeling of the complete process from the beginning of the collapse until the formation of the protostar. We solve the frequency-dependent radiative transfer equation without any diffusion approximation, using realistic dust and gas opacities. Our results show that the properties of the protostar are essentially independent of the initial conditions, which had previously only been confirmed for higher mass ranges. For very low-mass initial conditions, however, we find that the first core phase of the collapse shows some significant differences in the time evolution, with the first core lifetime increasing dramatically because of the reduced accretion rate from the surrounding envelope. We consider the observational implications of this. We also investigate the opposite case of a collapse without any first core phase, which may occur for very unstable initial conditions. In the Appendix, we describe a severe numerical problem that causes an unphysical expansion after the formation of the protostar, which may affect other attempts at similar calculations of self-gravitational collapse. We explain the origin of the unphysical behavior and present a solution that can be used in similar investigations.

DOI： 10.3847/1538-4357/aaee81

Web of Science

Scopus

Publisher：Astronomy and Astrophysics  

More than a half of the asteroids in the main belt have irregular shapes with ratios of the minor to major axis lengths of less than 0.6. One of the mechanisms that create such shapes is collisions between asteroids. The relationship between the shapes of collisional outcomes and impact conditions such as impact velocities may provide information on the collisional environments and its evolutionary stages when those asteroids are created. In this study, we perform numerical simulations of collisional destruction of asteroids with radii 50 km and subsequent gravitational reaccumulation using smoothed-particle hydrodynamics for elastic dynamics with self-gravity, a model of rock fractures, and a model of friction in completely damaged rock. We systematically vary the impact velocity from 50 to 400 m s ^-1 and the impact angle from 5° to 45°. We investigate shapes of the largest remnants resulting from collisional simulations. As a result, various shapes (bilobed, spherical, flat, elongated, and hemispherical shapes) are formed through equal-mass and low-velocity (50-400 m s ^-1 ) impacts. We clarify a range of the impact angle and velocity to form each shape. Our results indicate that irregular shapes, especially flat shapes, of asteroids with diameters larger than 80 km are likely to be formed through similar-mass and low-velocity impacts, which are likely to occur in the primordial environment prior to the formation of Jupiter.

DOI： 10.1051/0004-6361/201833227

Web of Science

Scopus

Publisher：Monthly Notices of the Royal Astronomical Society  

Recent numerical simulations of magnetized accretion discs show that the radial-azimuthal component of the stress tensor due to the magnetorotational instability is well represented by a power-law function of the gas pressure rather than a linear relation that has been used in most of the accretion disc studies. The exponent of this power-law function that depends on the net flux of the imposed magnetic field is reported in the range between zero and unity. However, the physical consequences of this power-law stress-pressure relation within the framework of the standard disc model have not been explored so far. In this study, the structure of an accretion disc with a power-law stress-pressure relation is studied using analytical solutions in the steady state and time-dependent cases. The derived solutions are applicable to different accreting systems, and as an illustrative example, we explore structure of protoplanetary discs using these solutions. We show that the slopes of the radial surface density and temperature distributions become steeper with decreasing the stress exponent. However, if the disc opacity is dominated by icy grains and value of the stress exponent is less than about 0.5, the surface density and temperature profiles become so steep that make them unreliable. We also obtain analytical solutions for the protoplanetary discs that are irradiated by the host star. Using these solutions, we find that the effect of the irradiation becomes more significant with decreasing the stress exponent.

DOI： 10.1093/mnras/sty2656

Web of Science

Scopus

Publisher：Astrophysical Journal Letters  

Pairs of azimuthal intensity decrements at near-symmetric locations have been seen in a number of protoplanetary disks. They are most commonly interpreted as the two shadows cast by a highly misaligned inner disk. Direct evidence of such an inner disk, however, remains largely illusive, except in rare cases. In 2012, a pair of such shadows were discovered in scattered-light observations of the near face-on disk around 2MASS J16042165-2130284, a transitional object with a cavity ∼60 au in radius. The star itself is a "dipper," with quasi-periodic dimming events on its light curve, commonly hypothesized as caused by extinctions by transiting dusty structures in the inner disk. Here, we report the detection of a gas disk inside the cavity using Atacama Large Millimeter/submillimeter Array (ALMA) observations with ∼0.″2 angular resolution. A twisted butterfly pattern is found in the moment 1 map of the CO (3-2) emission line toward the center, which is the key signature of a high misalignment between the inner and outer disks. In addition, the counterparts of the shadows are seen in both dust continuum emission and gas emission maps, consistent with these regions being cooler than their surroundings. Our findings strongly support the hypothesized misaligned inner disk origin of the shadows in the J1604-2130 disk. Finally, the inclination of the inner disk would be close to -45° in contrast with 45°; it is possible that its internal asymmetric structures cause the variations on the light curve of the host star.

DOI： 10.3847/2041-8213/aae88b

Web of Science

Scopus

Publisher：Astrophysical Journal  

The effect of misalignment between the magnetic field B and the angular momentum Jang of molecular cloud cores on the angular momentum evolution during the gravitational collapse is investigated by ideal and non-ideal MHD simulations. For the non-ideal effect, we consider the ohmic and ambipolar diffusion. Previous studies that considered the misalignment reported qualitatively contradicting results. Magnetic braking was reported as being either strengthened or weakened by misalignment in different studies. We conducted simulations of cloud core collapse by varying the stability parameter α (the ratio of the thermal to gravitational energy of the core) with and without including magnetic diffusion. The non-ideal MHD simulations show the central angular momentum of the core, with θ=0° (Jang B) being always greater than that with θ=90° (Jang B), independently of α, meaning that circumstellar disks form more easily in a core with θ=0°. The ideal MHD simulations, in contrast, show the central angular momentum of the core with θ=90° being greater than with θ=0° for small α and smaller for large α. Inspection of the angular momentum evolution of the fluid elements reveals three mechanisms contributing to the evolution of the angular momentum: (i) magnetic braking in the isothermal collapse phase, (ii) selective accretion of the rapidly (for θ = 90°) or slowly (for θ = 0°) rotating fluid elements to the central region, and (iii) magnetic braking in the first core and the disk. The difference between the ideal and non-ideal simulations arises from the different efficiencies of (iii).

DOI： 10.3847/1538-4357/aae4dc

Open Access

Web of Science

Scopus

Publisher：Publications of the Astronomical Society of Japan  

We present Nobeyama 45 m telescope C¹⁸O, ¹³CO, and ¹²CO(1-0) mapping observations towards an interstellar filament in the Taurus molecular cloud. We investigate the gas velocity structure along the filament and in its surrounding parent cloud. The filament is detected in the optically thin C¹⁸O emission as a single velocity component with a ∼1 pc long, ∼0.06 pc wide structure. The C¹⁸O emission traces dust column densities larger than ∼5 × 10²¹ cm⁻². The line-of-sight (LOS) velocity fluctuates along the filament crest with an average amplitude of ∼0.2 km s⁻¹. The ¹³CO and ¹²CO integrated intensity maps show spatially extended emission around the elongated filament. We identify three extended structures with LOS velocities redshifted and blueshifted with respect to the average velocity of the filament identified in C¹⁸O. Based on combined analyses of velocity-integrated channel maps and intensity variations of the optically thick ¹²CO spectra on and off the filament, we propose a three-dimensional structure of the cloud surrounding the filament. We further suggest a multi-interaction scenario where sheet-like extended structures interact, in space and time, with the filament and are responsible for its compression and/or disruption, playing an important role in the star formation history of the filament. We also identify, towards the same field, a very faint filament showing a velocity field compatible with the filament formation process proposed by Inoue et al. (2018, PASJ, 70, S53), where a filament is formed due to convergence of a flow of matter generated by the bending of the ambient magnetic field structure induced by an interstellar shock compression.

DOI： 10.1093/pasj/psy095

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Scopus

Publisher：Astrophysical Journal  

N49 (LHA 120-N49) is a bright X-ray supernova remnant (SNR) in the Large Magellanic Cloud. We present new ¹²CO (J = 1-0, 3-2), H i, and 1.4 GHz radio continuum observations of the SNR N49 using Mopra, ASTE, ALMA, and ATCA. We have newly identified three H i clouds using ATCA with an angular resolution of ∼20″: one associated with the SNR and the others located in front of the SNR. Both the CO and H i clouds in the velocity range from 281 to 291 km s^-1 are spatially correlated with both the soft X-rays (0.2-1.2 keV) and the hard X-rays (2.0-7.0 keV) of N49 on a ∼10 pc scale. CO 3-2/1-0 intensity ratios indicate higher values of the CO cloud toward the SNR shell with an angular resolution of ∼45″, and thus a strong interaction was suggested. Using the ALMA, we have spatially resolved CO clumps embedded within or along the southeastern rim of N49 with an angular resolution of ∼3″. Three of the CO clumps are rim brightened on a 0.7-2 pc scale in both hard X-rays and the radio continuum: this provides further evidence for dynamical interactions between the CO clumps and the SNR shock wave. The enhancement of the radio synchrotron radiation can be understood in terms of magnetic field amplification around the CO clumps via a shock-cloud interaction. We also present a possible scenario in which the recombining plasma that dominates the hard X-rays from N49 was formed via thermal conduction between the SNR shock waves and the cold/dense molecular clumps.

DOI： 10.3847/1538-4357/aacfff

Web of Science

Scopus

Publisher：Astrophysical Journal  

We report ALMA Cycle 3 observations in CO isotopes toward a dense core, MC27/L1521F in Taurus, which is considered to be at an early stage of multiple star formation in a turbulent environment. Although most of the high-density parts of this core are considered to be as cold as ∼10 K, high-angular resolution (∼20 au) observations in ¹²CO (J = 3-2) revealed complex warm (>15-60 K) filamentary/clumpy structures with the sizes from a few tens of astronomical units to ∼1000 au. The interferometric observations of ¹³CO and C¹⁸O show that the densest part with arc-like morphologies associated with the previously identified protostar and condensations are slightly redshifted from the systemic velocity of the core. We suggest that the warm CO clouds may be consequences of shock heating induced by interactions among the different density/velocity components that originated from the turbulent motions in the core. However, such a small-scale and fast turbulent motion does not correspond to a simple extension of the line-width-size relation (i.e., Larson's law), and thus the actual origin remains to be studied. The high-angular resolution CO observations are expected to be essential in detecting small-scale turbulent motions in dense cores and to investigate protostar formation therein.

DOI： 10.3847/1538-4357/aac898

Web of Science

Scopus

Publisher：Astrophysical Journal  

We present the results of dust emission polarization measurements of Ophiuchus-B (Oph-B) carried out using the Submillimetre Common-User Bolometer Array 2 (SCUBA-2) camera with its associated polarimeter (POL-2) on the James Clerk Maxwell Telescope in Hawaii. This work is part of the B-fields in Star-forming Region Observations survey initiated to understand the role of magnetic fields in star formation for nearby star-forming molecular clouds. We present a first look at the geometry and strength of magnetic fields in Oph-B. The field geometry is traced over ∼0.2 pc, with clear detection of both of the sub-clumps of Oph-B. The field pattern appears significantly disordered in sub-clump Oph-B1. The field geometry in Oph-B2 is more ordered, with a tendency to be along the major axis of the clump, parallel to the filamentary structure within which it lies. The degree of polarization decreases systematically toward the dense core material in the two sub-clumps. The field lines in the lower density material along the periphery are smoothly joined to the large-scale magnetic fields probed by NIR polarization observations. We estimated a magnetic field strength of 630 ± 410 μG in the Oph-B2 sub-clump using a Davis-Chandrasekhar-Fermi analysis. With this magnetic field strength, we find a mass-to-flux ratio λ = 1.6 ± 1.1, which suggests that the Oph-B2 clump is slightly magnetically supercritical.

DOI： 10.3847/1538-4357/aac4a6

Open Access

Web of Science

Scopus

Publisher：Monthly Notices of the Royal Astronomical Society  

Gas giant planets may form early on during the evolution of protostellar discs, while these are relatively massive. We study how Jupiter-mass planet-seeds (termed protoplanets) evolve in massive, but gravitationally stable (Q ≳ 1.5), discs using radiative hydrodynamic simulations. We find that the protoplanet initially migrates inwards rapidly, until it opens up a gap in the disc. Thereafter, it either continues to migrate inwards on a much longer time-scale or starts migrating outwards. Outward migration occurs when the protoplanet resides within a gap with gravitationally unstable edges, as a high fraction of the accreted gas is high angular momentum gas from outside the protoplanet's orbit. The effect of radiative heating from the protoplanet is critical in determining the direction of the migration and the eccentricity of the protoplanet. Gap opening is facilitated by efficient cooling that may not be captured by the commonly used β-cooling approximation. The protoplanet initially accretes at a high rate (~10^-3 M<inf>J</inf> yr^-1), and its accretion luminosity could be a few tenths of the host star's luminosity, making the protoplanet easily observable (albeit only for a short time). Due to the high gas accretion rate, the protoplanet generally grows above the deuterium-burning mass-limit. Protoplanet radiative feedback reduces its mass growth so that its final mass is near the brown dwarfplanet boundary. The fate of a young planet-seed is diverse and could vary from a gas giant planet on a circular orbit at a few au from the central star to a brown dwarf on an eccentric, wide orbit.

DOI： 10.1093/mnras/sty827

Web of Science

Scopus

Publisher：Astronomical Journal  

We develop a new numerical scheme for solving the radiative transfer equation in a spherically symmetric system. This scheme does not rely on any kind of diffusion approximation, and it is accurate for optically thin, thick, and intermediate systems. In the limit of a homogeneously distributed extinction coefficient, our method is very accurate and exceptionally fast. We combine this fast method with a slower but more generally applicable method to describe realistic problems. We perform various test calculations, including a simplified protostellar collapse simulation. We also discuss possible future improvements.

DOI： 10.3847/1538-3881/aac023

Web of Science

Scopus

Publisher：Astrophysical Journal  

We present 850 μm imaging polarimetry data of the ρ Oph-A core taken with the Submillimeter Common-User Bolometer Array-2 (SCUBA-2) and its polarimeter (POL-2) as part of our ongoing survey project, -fields In STar forming RegiOns (BISTRO). The polarization vectors are used to identify the orientation of the magnetic field projected on the plane of the sky at a resolution of 0.01 pc. We identify 10 subregions with distinct polarization fractions and angles in the 0.2 pc ρ Oph-A core; some of them can be part of a coherent magnetic field structure in the ρ Oph region. The results are consistent with previous observations of the brightest regions of ρ Oph-A, where the degrees of polarization are at a level of a few percent, but our data reveal for the first time the magnetic field structures in the fainter regions surrounding the core where the degree of polarization is much higher (>5%). A comparison with previous near-infrared polarimetric data shows that there are several magnetic field components that are consistent at near-infrared and submillimeter wavelengths. Using the Davis-Chandrasekhar-Fermi method, we also derive magnetic field strengths in several subcore regions, which range from approximately 0.2 to 5 mG. We also find a correlation between the magnetic field orientations projected on the sky and the core centroid velocity components.

DOI： 10.3847/1538-4357/aabd82

Web of Science

Scopus

Publisher：Astrophysical Journal  

We present ALMA 0.87 mm continuum, HCO⁺ J = 4-3 emission line, and CO J = 3-2 emission line data of the disk of material around the young, Sun-like star PDS 70. These data reveal the existence of a possible two-component transitional disk system with a radial dust gap of 0.″42 ±0.″05, an azimuthal gap in the HCO⁺ J = 4-3 moment zero map, as well as two bridge-like features in the gas data. Interestingly these features in the gas disk have no analog in the dust disk making them of particular interest. We modeled the dust disk using the Monte Carlo radiative transfer code HOCHUNK3D using a two-disk component. We find that there is a radial gap that extends from 15 to 60 au in all grain sizes, which differs from previous work.

DOI： 10.3847/1538-4357/aaba7c

Web of Science

Scopus

Publisher：Publications of the Astronomical Society of Japan  

Recent millimeter/submillimeter observations towards nearby galaxies have started to map the whole disk and to identify giant molecular clouds (GMCs) even in the regions between galactic spiral structures. Observed variations of GMC mass functions in different galactic environments indicates that massive GMCs preferentially reside along galactic spiral structures whereas inter-arm regions have many small GMCs. Based on the phase transition dynamics from magnetized warm neutral medium to molecular clouds, Kobayashi et al. (2017, ApJ, 836, 175) proposes a semi-analytical evolutionary description for GMC mass functions including a cloud-cloud collision (CCC) process. Their results show that CCC is less dominant in shaping the mass function of GMCs than the accretion of dense HI gas driven by the propagation of supersonic shock waves. However, their formulation does not take into account the possible enhancement of star formation by CCC. Millimeter/submillimeter observations within the Milky Way indicate the importance of CCC in the formation of star clusters and massive stars. In this article, we reformulate the time-evolution equation largelymodified from Kobayashi et al. (2017, ApJ, 836, 175) so that we additionally compute star formation subsequently taking place in CCC clouds. Our results suggest that, although CCC events between smaller clouds are more frequent than the ones between massive GMCs, CCC-driven star formation is mostly driven by massive GMCs ≲ 10^5.5 M<inf>⊙</inf> (where M<inf>⊙</inf> is the solar mass). The resultant cumulative CCC-driven star formation may amount to a few 10 percent of the total star formation in the Milky Way and nearby galaxies.

DOI： 10.1093/pasj/psy018

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Scopus

Publisher：Publications of the Astronomical Society of Japan  

Recent observations suggest an that intensive molecular cloud collision can trigger massive star/cluster formation. The most important physical process caused by the collision is a shock compression. In this paper, the influence of a shock wave on the evolution of a molecular cloud is studied numerically by using isothermal magnetohydrodynamics simulations with the effect of self-gravity. Adaptive mesh refinement and sink particle techniques are used to follow the long-time evolution of the shocked cloud. We find that the shock compression of a turbulent inhomogeneous molecular cloud creates massive filaments, which lie perpendicularly to the background magnetic field, as we have pointed out in a previous paper. The massive filament shows global collapse along the filament, which feeds a sink particle located at the collapse center. We observe a high accretion rate M<inf>acc</inf> > 10^-4 M<inf>⊙</inf> yr^-1 that is high enough to allow the formation of even O-type stars. The most massive sink particle achieves M > 50M<inf>⊙</inf> in a fewtimes 10⁵ yr after the onset of the filament collapse.

DOI： 10.1093/pasj/psx089

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Scopus

DOI： 10.3847/1538-4357/aaafc8

Web of Science

Publisher：Monthly Notices of the Royal Astronomical Society  

Our paper 'A revised condition for self-gravitational fragmentation of protoplanetary discs' was published in MNRAS 458, 3597-3612 (2016). The value of βnet in Fig. 10 of the original paper was incorrect. When we calculated βnet in the figure, we underestimated the equilibrium temperature under the irradiation from the central star resulting in the underestimation of βnet. The temperature of the spiral arm is similar to that of the equilibrium temperature and the corrected βnet is ≳10. This indicates that the thermal energy provided by the compressional heating has already been emitted away from the spiral arm at the epoch of Fig. 10. Thus, βnet at the time of Fig. 10 is not appropriate for the discussion for the relation between the fragmentation of the disk and the cooling time. Therefore, we provide the alternative figure here (Fig. 1). The quantities in Fig. 1 are calculated from the same simulation which was used for Fig. 10 of the original paper but at t = 2714 yr. Since the left panels in Fig. 17 of the original paper correspond to the top and bottom panels of the original Fig. 10, we should also replace it with this new figure. As shown in the figures, βnet is ≲6. This is the value of fragmenting region in the model S265k005 (Fig. 8 in the original paper). All the discussions and conclusions in the original paper remain unchanged because the disk fragmentation is not observed in this model although βnet is smaller than that of the model S265k005 in which the fragmentation occurs. However, we revoke the sentence 'As a result, the minimum normalized cooling time in the spiral arm is about unity.' in the second paragraph of Section 3.4 of the original paper. Acknowledgements: We would like to thank Shigenobu Hirose for comparison of simulation results, which drew our attention to our error. 'Figure Presented'. This paper has been typeset from a TEX/LATEX file prepared by the author.

DOI： 10.1093/mnras/stx2495

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Scopus

Publisher：Publications of the Astronomical Society of Japan  

We perform non-linear simulation of secular gravitational instability (GI) in protoplanetary disks, which has been proposed as a mechanism of planetesimal and multiple ring formation. Since the timescale of the growth of the secular GI is much longer than the Keplerian rotation period, we develop a new numerical scheme for a long-term calculation utilizing the concept of symplectic integration. With our new scheme, we first investigate the non-linear development of the secular GI in a disk without a pressure gradient in the initial state. We find that the surface density of dust increases by more than a factor of 100 while that of gas does not increase even by a factor of 2, which results in the formation of dust-dominated rings. A line mass of the dust ring tends to be very close to the critical line mass of a self-gravitating isothermal filament. Our results indicate that the non-linear growth of the secular GI provides a powerful mechanism to concentrate the dust. We also find that the dust ring formed via the non-linear growth of the secular GI migrates inward with a low velocity, which is driven by the self-gravity of the ring. We give a semi-analytical expression for the inward migration speed of the dusty ring.

DOI： 10.1093/pasj/psx143

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Scopus

Publisher：Proceedings of the International Astronomical Union  

DOI： 10.1017/S1743921319003247

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Publisher：Publications of the Astronomical Society of Japan  

We perform three-dimensional radiation non-ideal magnetohydrodynamics simulations and investigate the impact of the Hall effect on the angular momentum evolution in collapsing cloud cores in which the magnetic field B and angular momentum J<inf>ang</inf> are misaligned with each other. We find that the Hall effect noticeably changes the magnetic torques in the pseudo-disk, and strengthens and weakens the magnetic braking in cores with acute and obtuse relative angles between B and J<inf>ang</inf>, respectively. This suggests that the bimodal evolution of the disk size may occur in the early disk evolutionary phase even if B and J<inf>ang</inf> are randomly distributed. We show that a counter-rotating envelope forms in the upper envelope of the pseudo-disk in cloud coreswith obtuse relative angles. We also find that a counter-rotating region forms at the midplane of the pseudo-disk in cloud cores with acute relative angles. The former and latter types of counter-rotating envelopes may be associated with young stellar objects with large (r ~ 100 au) and small (r ≤ 10 au) disks, respectively.

DOI： 10.1093/pasj/psx113

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Scopus

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.3847/1538-4357/aa8e9e

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DOI： 10.3847/1538-4357/aa8e42

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DOI： 10.3847/1538-4357/aa8b62

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DOI： 10.3847/1538-4357/aa70a0

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.3847/1538-4357/aa6af7

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Scopus

Publisher：Astronomy and Astrophysics  

Context. Debris disks are important observational clues for understanding planetary-system formation process. In particular, faint warm debris disks may be related to late planet formation near 1 au. A systematic search of faint warm debris disks is necessary to reveal terrestrial planet formation. Aims. Faint warm debris disks show excess emission that peaks at mid-IR wavelengths. Thus we explore debris disks using the AKARI mid-IR all-sky point source catalog (PSC), a product of the second generation unbiased IR all-sky survey. Methods. We investigate IR excess emission for 678 isolated main-sequence stars for which there are 18 μm detections in the AKARI mid-IR all-sky catalog by comparing their fluxes with the predicted fluxes of the photospheres based on optical to near-IR fluxes and model spectra. The near-IR fluxes are first taken from the 2MASS PSC. However, 286 stars with Ks < 4.5 in our sample have large flux errors in the 2MASS photometry due to saturation. Thus we have measured accurate J, H, and Ks band fluxes, applying neutral density (ND) filters for Simultaneous InfraRed Imager for Unbiased Survey (SIRIUS) on IRSF, the φ1.4 m near-IR telescope in South Africa, and improved the flux accuracy from 14% to 1.8% on average. Results. We identified 53 debris-disk candidates including eight new detections from our sample of 678 main-sequence stars. The detection rate of debris disks for this work is ~8%, which is comparable with those in previous works by Spitzer and Herschel. Conclusions. The importance of this study is the detection of faint warm debris disks around nearby field stars. At least nine objects have a large amount of dust for their ages, which cannot be explained by the conventional steady-state collisional cascade model.

DOI： 10.1051/0004-6361/201526215

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.3847/1538-4357/aa6a1c

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DOI： 10.1016/j.jcp.2016.12.026

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DOI： 10.3847/1538-4357/836/2/175

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Scopus

DOI： 10.1017/S1743921316012400

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Memorie Della Societa Astronomica Italiana Journal of the Italian Astronomical Society  

Recent theoretical investigation provides a picture of molecular cloud formation in interacting shells or bubbles in the Galactic disk. Filametary molecular clouds are ubiquitously formed in magnetized dense shells created by expanding bubbles. Star formation starts in dense cores in filamentary molecular cloud, once the mass per unit length of the filament exceeds the critical line-mass. An integrated picture of the cloud formation explains many observational properties such as, cloud-to-cloud velocity dispersions, molecular cloud mass function, dense core mass function, and very low star formation efficiencies in the Galactic disk, as well as the reason for the acceleration of star formation in each star forming region.

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Memorie Della Societa Astronomica Italiana Journal of the Italian Astronomical Society  

We formulate the evolution equation for the giant molecular cloud (GMC) mass functions including self-growth of GMCs through the thermal instability, self-dispersal due to massive stars born in GMCs, cloud-cloud collisions (CCCs), and gas resurrection that replenishes the minimum-mass GMC population. The computed time evolutions obtained from this formulation suggest that the slope of GMC mass function in the mass range < 103⁵⁵ M<inf>O</inf> is governed by the ratio of GMC formation timescale to its dispersal timescale, and that the CCC process modifies only the massive end of the mass function. Our results also suggest that most of the dispersed gas contributes to the mass growth of pre-existing GMCs in arm regions whereas less than 60 per cent contributes in inter-arm regions.

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CiNii Research

DOI： 10.1088/1742-6596/837/1/012009

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1093/mnras/stw1994

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.3847/0004-6256/152/6/184

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DOI： 10.1093/mnras/stw2256

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Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)  

Disc formation in strongly magnetized cloud cores is investigated using a three-dimensional magnetohydrodynamic simulation with a focus on the effects of the initial cloud stability and the mass accretion rate. The initial cloud stability greatly alters the disc formation process even for prestellar clouds with the same mass-to-flux ratio. A high mass accretion rate on to the disc-forming region is realized in initially unstable clouds, and a large angular momentum is introduced into the circumstellar region in a short time. The region around the protostar has both a thin infalling envelope and a weak magnetic field, which both weaken the effect of magnetic braking. The growth of the rotation-supported disc is promoted in such unstable clouds. Conversely, clouds in an initially near-equilibrium state show lower accretion rates of mass and angular momentum. The angular momentum is transported to the outer envelope before protostar formation. After protostar formation, the circumstellar region has a thick infalling envelope and a strong magnetic field that effectively brakes the disc. As a result, disc formation is suppressed when the initial cloud is in a nearly stable state. The density distribution of the initial cloud also affects the disc formation process. Disc growth strongly depends on the initial conditions when the prestellar cloud has a uniform density, whereas there is no significant difference in the disc formation process in prestellar clouds with non-uniform densities.

DOI： 10.1093/mnras/stw2256

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.3847/0004-637X/826/1/26

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1007/s10712-016-9361-7

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.3847/0004-637X/821/1/3

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.jcp.2015.12.030

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.astropartphys.2015.06.001

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1017/S174392131600836X

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1017/S1743921316007262

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1088/0004-637X/815/1/78

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1093/pasj/psv098

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1088/0004-637X/809/2/125

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1051/0004-6361/201425584

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Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)  

We describe an overall picture of galactic-scale star formation. Recent high-resolution magneto-hydrodynamical simulations of two-fluid dynamics with cooling, heating, and thermal conduction have shown that the formation of molecular clouds requires multiple episodes of supersonic compression. This finding enables us to create a scenario in which molecular clouds form in interacting shells or bubbles on a galactic scale. First, we estimated the ensemble-averaged growth rate of molecular clouds on a timescale longer than a million years. Next, we performed radiation hydrodynamics simulations to evaluate the destruction rate of magnetized molecular clouds by the stellar far-ultraviolet radiation. We also investigated the resulting star formation efficiency within a cloud, which amounts to a low value (a few percent) if we adopt the power-law exponent ~2.5 for the mass distribution of stars in the cloud. We finally describe the time evolution of the mass function of molecular clouds on a long timescale (>1 Myr) and discuss the steady state exponent of the power-law slope in various environments.

DOI： 10.1051/0004-6361/201425584

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1051/0004-6361/201525636

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1093/mnrasl/slv031

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1088/0004-637X/800/1/47

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Language：English   Publishing type：Research paper (scientific journal)  

We investigate the structure of self-gravitating discs, their fragmentation and the evolution of the fragments (the clumps) using both an analytic approach and three-dimensional radiation hydrodynamics simulations starting from molecular cores. The simulations show that non-local radiative transfer determines the disc temperature. We find the disc structure is well described by an analytical model of a quasi-steady self-gravitating disc with radial radiative transfer. Because the radiative process is not local and radiation from the interstellar medium cannot be ignored, the local radiative cooling is not balanced with the viscous heating in a massive disc around a low-mass star. In our simulations, there are cases in which the disc does not fragment even though it satisfies the fragmentation criterion based on disc cooling time (Q~1 and Ωtcool~1). This indicates that, at least, the criterion is not a sufficient condition for fragmentation. We determine the parameter range for the host cloud core in which disc fragmentation occurs. In addition, we show that the temperature evolution of the centre of the clump is close to that of typical first cores, and that the minimum initial mass of clumps is about a few Jupiter masses.

DOI： 10.1093/mnras/stu2160

Language：English   Publishing type：Research paper (scientific journal)  

The instability in protoplanetary disks due to gas-dust friction and self-gravity of gas and dust is investigated using linear analysis. In the case where the dust-to-gas ratio is enhanced and turbulence is weak, the instability grows, even in gravitationally stable disks, on a timescale of order 104-5 yr at a radius of order 100 AU. If we ignore the dynamical feedback from dust grains in the gas equation of motion, the instability reduces to the so-called "secular gravitational instability," which was investigated previously to be an instability of dust in a fixed background gas flow. In this work, we solve the equations of motion for both gas and dust consistently and find that long-wavelength perturbations are stable, in contrast to the secular gravitational instability in the simplified treatment. This may indicate that we should not neglect small terms in the equation of motion if the growth rate is small. The instability is expected to form ring structures in protoplanetary disks. The width of the ring formed at a radius of 100 AU is a few tens of AU. Therefore, the instability is a candidate for the formation mechanism of observed ring-like structures in disks. Another aspect of the instability is the accumulation of dust grains, and hence the instability may play an important role in the formation of planetesimals, rocky protoplanets, and cores of gas giants located at radii ~100 AU. If these objects survive the dispersal of the gaseous component of the disk, they may be the origin of debris disks.

DOI： 10.1088/0004-637X/794/1/55

Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)  

In this article, we present a state-of-the-art algorithm for solving the relativistic viscous hydrodynamics equation with the QCD equation of state. The numerical method is based on the second-order Godunov method and has less numerical dissipation, which is crucial in describing of quark-gluon plasma in high-energy heavy-ion collisions. We apply the algorithm to several numerical test problems such as sound wave propagation, shock tube and blast wave problems. In sound wave propagation, the intrinsic numerical viscosity is measured and its explicit expression is shown, which is the second-order of spatial resolution both in the presence and absence of physical viscosity. The expression of the numerical viscosity can be used to determine the maximum cell size in order to accurately measure the effect of physical viscosity in the numerical simulation.

DOI： 10.1016/j.jcp.2013.08.047

Language：English   Publishing type：Research paper (scientific journal)  

We investigate the formation of close-in terrestrial planets from planetary embryos under the influence of a hot Jupiter (HJ) using gravitational N-body simulations that include gravitational interactions between the gas disk and the terrestrial planet (e.g., type I migration). Our simulations show that several terrestrial planets efficiently form outside the orbit of the HJ, making a chain of planets, and all of them gravitationally interact directly or indirectly with the HJ through resonance, which leads to inward migration of the HJ. We call this mechanism of induced migration of the HJ "crowding-out." The HJ is eventually lost through collision with the central star, and only several terrestrial planets remain. We also find that the efficiency of the crowding-out effect depends on the model parameters; for example, the heavier the disk is, the more efficient the crowding-out is. When planet formation occurs in a massive disk, the HJ can be lost to the central star and is never observed. On the other hand, for a less massive disk, the HJ and terrestrial planets can coexist; however, the companion planets may be below the detection limit of current observations. In both cases, systems with a HJ and terrestrial planets have little chance of detection. Therefore, our model naturally explains the lack of companion planets in HJ systems regardless of the disk mass. In effect, our model provides a theoretical prediction for future observations; additional planets can be discovered just outside the HJ, and their masses should generally be small.

DOI： 10.1088/2041-8205/778/1/L9

Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)  

Essential physical processes in the formation of protostars and protoplanetary disks are described. Recent advances in non-ideal magnetohydrodynamics simulations, which cover a huge dynamic range from molecular cloud core density (10^4/cc) to stellar density (10^22/cc) in a self-consistent manner, enable us to study the realistic evolution of the magnetic field and rotation of protostars and the dynamics of outflows and jets.

DOI： 10.1093/ptep/pts024

Open Access

Language：English   Publishing type：Research paper (scientific journal)  

We perform a linear perturbation analysis of expanding shells driven by expansions of H II regions. The ambient gas is assumed to be uniform. As an unperturbed state, we develop a semi-analytic method for deriving the time evolution of the density profile across the thickness. It is found that the time evolution of the density profile can be divided into three evolutionary phases: deceleration-dominated, intermediate, and self-gravity-dominated phases. The density peak moves relatively from the shock front to the contact discontinuity as the shell expands. We perform a linear analysis taking into account the asymmetric density profile obtained by the semi-analytic method, and imposing the boundary conditions for the shock front and the contact discontinuity while the evolutionary effect of the shell is neglected. It is found that the growth rate is enhanced compared with previous studies based on the thin-shell approximation. This is due to the boundary effect of the contact discontinuity and asymmetric density profile that were not taken into account in previous works.

DOI： 10.1088/0004-637X/733/1/16

Language：English   Publishing type：Research paper (scientific journal)  

We investigate the gravitational fragmentation of expanding shells driven by H II regions using the three-dimensional Lagrangian simulation codes based on the Riemann solver, called Godunov smoothed particle hydrodynamics. The ambient gas is assumed to be uniform. In order to attain high resolution to resolve the geometrically thin dense shell, we calculate not the whole but a part of the shell. We find that perturbations begin to grow earlier than predicted by linear analysis under the thin-shell approximation. The wavenumber of the most unstable mode is larger than that in the thin-shell linear analysis. The development of the gravitational instability is accompanied by the significant deformation of the contact discontinuity. These results are consistent with a linear analysis presented by Iwasaki et al. that have taken into account the density profile across the thickness and approximate shock and contact discontinuity boundary conditions. We derive useful analytic formulae for the fragment scale and the epoch when the gravitational instability begins to grow.

DOI： 10.1088/0004-637X/733/1/17

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

The formation and evolution of the circumstellar disk in unmagnetized molecular clouds is investigated using three-dimensional hydrodynamic simulations from the prestellar core until the end of the main accretion phase. In collapsing cloud cores, the first (adiabatic) core with a size of gsim3 AU forms prior to the formation of the protostar. At its formation, the first core has a thick disk-like structure and is mainly supported by the thermal pressure. After the protostar formation, it decreases the thickness gradually and becomes supported by the centrifugal force. We found that the first core is a precursor of the circumstellar disk with a size of >3 AU. This means that unmagnetized protoplanetary disk smaller than <3 AU does not exist. Reflecting the thermodynamics of the collapsing gas, at the protostar formation epoch, the first core (or the circumstellar disk) has a mass of ~0.005-0.1 M sun, while the protostar has a mass of ~10-3 M sun. Thus, just after the protostar formation, the circumstellar disk is about 10-100 times more massive than the protostar. In the main accretion phase that lasts for ~105 yr, the circumstellar disk mass initially tends to dominate the protostellar mass. Such a massive disk is unstable to gravitational instability and tends to show fragmentation. Our calculations indicate that the low-mass companions may form in the circumstellar disk in the main accretion phase. In addition, the mass accretion rate onto the protostar shows a strong time variability that is caused by the torque from the low-mass companions and/or the spiral arms in the circumstellar disk. Such variability provides an important signature for detecting the substellar mass companion in the circumstellar disk around very young protostars.

Language：English   Publishing type：Research paper (scientific journal)  

In this paper, we study the Cauchy problem of the linearized kinetic equations for the models of Marle and Anderson-Witting, and compare these dispersion relations with the 14-moment theory. First, we propose a modification of the Marle model to improve the resultant transport coefficients in accordance with those obtained by the full Boltzmann equation. Using the modified Marle model and Anderson-Witting model, we calculate dispersion relations that are kinetically correct within the validity of the BGK approximation. The 14-moment theory that includes the time derivative of dissipation currents has a causal structure, in contrast to the acausal first-order Chapman-Enskog approximation. However, the dispersion relation of the 14-moment theory does not accurately describe the result of the kinetic equation. Thus, our calculation indicates that keeping these second-order terms does not simply correspond to improving the physical description of the relativistic hydrodynamics.

DOI： 10.1016/j.physa.2010.06.021

Language：English   Publishing type：Research paper (scientific journal)  

Recent gamma-ray observations of middle-aged supernova remnants revealed a mysterious broken power-law spectrum. Using three-dimensional magnetohydrodynamic simulations, we show that the interaction between a supernova blast wave and interstellar clouds formed by thermal instability generates multiple reflected shocks. The typical Mach numbers of the reflected shocks are shown to be Msime 2 depending on the density contrast between the diffuse intercloud gas and clouds. These secondary shocks can further energize cosmic-ray particles originally accelerated at the blast-wave shock. This "two-step" acceleration scenario reproduces the observed gamma-ray spectrum and predicts the high-energy spectral index ranging approximately from 3 to 4.

Language：English   Publishing type：Research paper (scientific journal)  

We analyze the physical processes of gap formation in an inviscid protoplanetary disk with an embedded protoplanet using a two-dimensional local shearing-sheet model. The spiral density wave launched by the planet shocks and the angular momentum carried by the wave is transferred to the background flow. The exchange of the angular momentum can affect the mass flux in the vicinity of the planet to form an underdense region, or gap, around the planetary orbit. We first perform weakly nonlinear analyses to show that the specific vorticity formed by shock dissipation of the density wave can be a source of mass flux in the vicinity of the planet and that the gap can be opened even for low-mass planets unless the migration of the planet is substantial. We then perform high-resolution numerical simulations to check analytic consideration. By comparing the gap-opening timescale and type I migration timescale, we propose a criterion for the formation of underdense region around the planetary orbit that is qualitatively different from previous studies. The minimum mass required for the planet to form a dip is twice as small as previous studies if we incorporate the standard values of type I migration timescale, but it can be much smaller if there is a location in the disk where type I migration is halted.

Language：English   Publishing type：Research paper (scientific journal)  

By constructing a global model based on three-dimensional local magnetohydrodynamical simulations, we show that the disk wind driven by magnetorotational instability (MRI) plays a significant role in the dispersal of the gas component of protoplanetary disks. Because the mass loss timescale of the MRI-driven disk winds is proportional to the local Keplerian rotation period, a gas disk dynamically evaporates from the inner region, possibly creating a gradually expanding inner hole, while a sizable amount of the gas remains in the outer region. The disk wind is highly time dependent with a quasi-periodicity of several times the Keplerian rotation period at each radius, which will be observed as the time variability of protostar-protoplanetary disk systems. These features persistently hold even if a dead zone exists because the disk winds are driven from the surface regions where ionizing cosmic rays and high energy photons can penetrate. Moreover, the predicted inside-out clearing significantly suppresses the infall of boulders to a central star and the type I migration of proto-planets, which are favorable for the formation and survival of planets.

Language：English   Publishing type：Research paper (scientific journal)  

By constructing a global model based on three-dimensional local magnetohydrodynamical simulations, we show that the disk wind driven by magnetorotational instability (MRI) plays a significant role in the dispersal of the gas component of protoplanetary disks. Because the mass loss timescale of the MRI-driven disk winds is proportional to the local Keplerian rotation period, a gas disk dynamically evaporates from the inner region, possibly creating a gradually expanding inner hole, while a sizable amount of the gas remains in the outer region. The disk wind is highly time dependent with a quasi-periodicity of several times the Keplerian rotation period at each radius, which will be observed as the time variability of protostar-protoplanetary disk systems. These features persistently hold even if a dead zone exists because the disk winds are driven from the surface regions where ionizing cosmic rays and high energy photons can penetrate. Moreover, the predicted inside-out clearing significantly suppresses the infall of boulders to a central star and the type I migration of proto-planets, which are favorable for the formation and survival of planets.

Language：English   Publishing type：Research paper (scientific journal)  

We investigate the formation process of planetesimals from the dust layer by the gravitational instability in the gas disk using local N-body simulations. The gas is modeled as a background laminar flow. We study the formation process of planetesimals and its dependence on the strength of the gas drag. Our simulation results show that the formation process is divided into three stages qualitatively: the formation of wake-like density structures, the creation of planetesimal seeds, and their collisional growth. The linear analysis of the dissipative gravitational instability shows that the dust layer is secularly unstable although Toomre's Q value is larger than unity. However, in the initial stage, the growth time of the gravitational instability is longer than that of the dust sedimentation and the decrease in the velocity dispersion. Thus, the velocity dispersion decreases and the disk shrinks vertically. As the velocity dispersion becomes sufficiently small, the gravitational instability finally becomes dominant. Then wake-like density structures are formed by the gravitational instability. These structures fragment into planetesimal seeds. The seeds grow rapidly owing to mutual collisions.

Language：English   Publishing type：Research paper (scientific journal)  

We use resistive magnetohydrodynamical (MHD) simulations with the nested grid technique to study the formation of protoplanetary disks around protostars from molecular cloud cores that provide the realistic environments for planet formation. We find that gaseous planetary-mass objects are formed in the early evolutionary phase by gravitational instability in regions that are decoupled from the magnetic field and surrounded by the injection points of the MHD outflows during the formation phase of protoplanetary disks. Magnetic decoupling enables massive disks to form and these are subject to gravitational instability, even at ~10 AU. The frequent formation of planetary-mass objects in the disk suggests the possibility of constructing a hybrid planet formation scenario, where the rocky planets form later under the influence of the giant planets in the protoplanetary disk.

Language：English   Publishing type：Research paper (scientific journal)  

We investigate gas accretion on to a protoplanet, by considering the thermal effect of gas in three-dimensional hydrodynamical simulations, in which the wide region from a protoplanetary gas disc to a Jovian radius planet is resolved using the nested grid method. We estimate the mass accretion rate and growth time-scale of gas giant planets. The mass accretion rate increases with protoplanet mass for Mp < Mcri, while it becomes saturated or decreases for Mp > Mcri, where Mcri ≡ 0.036MJup(ap/1 au)0.75, and MJup and ap are the Jovian mass and the orbital radius, respectively. This accretion rate is typically two orders of magnitude smaller than that in two-dimensional simulations. The growth time-scale of a gas giant planet or the time-scale of the gas accretion on to the protoplanet is about 105 yr, that is two orders of magnitude shorter than the growth time-scale of the solid core. The thermal effects barely affect the mass accretion rate because the gravitational energy dominates the thermal energy around the protoplanet. The mass accretion rate obtained in our local simulations agrees quantitatively well with those obtained in global simulations with coarser spatial resolution. The mass accretion rate is mainly determined by the protoplanet mass and the property of the protoplanetary disc. We find that the mass accretion rate is correctly calculated when the Hill or Bondi radius is sufficiently resolved. Using the oligarchic growth of protoplanets, we discuss the formation time-scale of gas giant planets.

DOI： 10.1111/j.1365-2966.2010.16527.x

Language：English   Publishing type：Research paper (scientific journal)  

We study the linearized kinetic equation of relaxation model proposed by Bhatnagar, Gross and Krook [P. L. Bhatnagar, E. P. Gross and M. Krook, Phys. Rev. 94 (1954), 511] (also called BGK model) and solve the dispersion relation. Using the solution of the dispersion relation, we analyze the relaxation of the macroscopic mode and kinetic mode. Since the BGK model is not based on the expansion in the mean free path in contrast to the Chapman-Enskog expansion, the solution can describe the accurate relaxation of initial disturbance with any wavelength. This nonrelativistic analysis gives suggestions for our next work on the relativistic analysis of relaxation.

Language：English   Publishing type：Research paper (scientific journal)  

Numerical simulations for the non-linear development of Kelvin-Helmholtz instability in two different density layers have been performed with the particle-based method (Godunov SPH) developed by Inutsuka. The Godunov SPH can describe the Kelvin-Helmholtz instability even with a high-density contrast, while the standard SPH shows the absence of the instability across a density gradient. The interaction of a dense blob with a hot ambient medium has been performed also. The Godunov SPH describes the formation and evolution of the fingers due to the combinations of Rayleigh-Taylor, Richtmyer-Meshkov and Kelvin-Helmholtz instabilities. The blob test result coincides well with the results of the grid-based codes.
An inaccurate handling of a density gradient in the standard SPH has been pointed out as the direct reason of the absence of the instabilities. An unphysical force happens at the density gradient even in a pressure equilibrium, and repulses particles from the initial density discontinuity. Therefore, the initial perturbation damps, and a gap form at the discontinuity. The unphysical force has been studied in terms of the consistency of a numerical scheme. Contrary to the standard SPH, the momentum equation of the Godunov SPH does not use the particle approximation, and has been derived from the kernel convolution or a new Lagrangian function. The new Lagrangian function used in the Godunov SPH is more analogous to the real Lagrangian function for continuum. The momentum equation of the Godunov SPH has much better linear consistency, so the unphysical force is greatly reduced compared to the standard SPH in a high density contrast.

DOI： 10.1111/j.1365-2966.2010.16200.x

Language：English   Publishing type：Research paper (scientific journal)  

The fragmentation process in collapsing clouds with various metallicities is studied using three-dimensional nested-grid hydrodynamics. Initial clouds are specified by three parameters: cloud metallicity, initial rotation energy and initial cloud shape. For different combinations of these parameters, we calculate 480 models in total and study cloud evolution, fragmentation conditions, orbital separation and binary frequency. For the cloud to fragment during collapse, the initial angular momentum must be higher than a threshold value, which decreases with decreasing metallicity. Although the exact fragmentation conditions depend also on the initial cloud shape, this dependence is only modest. Our results indicate a higher binary frequency in lower metallicity gas. In particular, with the same median rotation parameter as in the solar neighbourhood, a majority of stars are born as members of binary/multiple systems for <10-4Zsolar. With initial mass <0.1Msolar, if fragments are ejected in embryo from the host clouds by multibody interaction, they evolve to substellar-mass objects. This provides a formation channel for low-mass stars in zero- or low-metallicity environments.

DOI： 10.1111/j.1365-2966.2009.15394.x

Language：English   Publishing type：Research paper (scientific journal)  

The gravitational instability of a dust layer is one of the scenarios for planetesimal formation. If the density of a dust layer becomes sufficiently high as a result of the sedimentation of dust grains toward the midplane of a protoplanetary disk, the layer becomes gravitationally unstable and spontaneously fragments into planetesimals. Using a shearing box method, we performed local N-body simulations of gravitational instability of a dust layer and subsequent coagulation without gas and investigated the basic formation process of planetesimals. In this paper, we adopted the accretion model as a collision model. A gravitationally bound pair of particles is replaced by a single particle with the total mass of the pair. This accretion model enables us to perform long-term and large-scale calculations. We confirmed that the formation process of planetesimals is the same as that in the previous paper with the rubble pile models. The formation process is divided into three stages: the formation of nonaxisymmetric structures; the creation of planetesimal seeds; and their collisional growth. We investigated the dependence of the planetesimal mass on the simulation domain size. We found that the mean mass of planetesimals formed in simulations is proportional to L 3/2 y , where Ly is the size of the computational domain in the direction of rotation. However, the mean mass of planetesimals is independent of Lx , where Lx is the size of the computational domain in the radial direction if Lx is sufficiently large. We presented the estimation formula of the planetesimal mass taking into account the simulation domain size.

Language：English   Publishing type：Research paper (scientific journal)  

Formation of interstellar clouds as a consequence of thermal instability is studied using two-dimensional two-fluid magnetohydrodynamic simulations. We consider the situation of converging, supersonic flows of warm neutral medium in the interstellar medium that generate a shocked slab of thermally unstable gas in which clouds form. We find, as speculated in Paper I, that in the shocked slab magnetic pressure dominates thermal pressure and the thermal instability grows in the isochorically cooling, thermally unstable slab that leads to the formation of H I clouds whose number density is typically n lsim 100 cm-3, even if the angle between magnetic field and converging flows is small. We also find that even if there is a large dispersion of magnetic field, evolution of the shocked slab is essentially determined by the angle between the mean magnetic field and converging flows. Thus, the direct formation of molecular clouds by piling up warm neutral medium does not seem to be a typical molecular cloud formation process, unless the direction of supersonic converging flows is biased to the orientation of mean magnetic field by some mechanism. However, when the angle is small, the H I shell generated as a result of converging flows is massive and possibly evolves into molecular clouds, provided gas in the massive H I shell is piled up again along the magnetic field line. We expect that another subsequent shock wave can again pile up the gas of the massive shell and produce a larger cloud. We thus emphasize the importance of multiple episodes of converging flows, as a typical formation process of molecular clouds.

Language：English   Publishing type：Research paper (scientific journal)  

Protostellar outflow is a star's first cry at the moment of birth. The outflows have an indispensable role in the formation of single stars because they carry off the excess angular momentum from the center of the shrinking gas cloud, and permit further collapse to form a star. On the other hand, a significant fraction of stars is supposedly born as binaries with circumbinary disks that are frequently observed. Here, we investigate the evolution of a magnetized rotating cloud using a three-dimensional resistive MHD nested-grid code, and show that the outflow is driven by the circumbinary disk and has an important role even in the binary formation. After the adiabatic core formation in the collapsing cloud core, the magnetic flux is significantly removed from the center of the cloud by the Ohmic dissipation. Since this removal makes the magnetic braking ineffective, the adiabatic core continuously acquires the angular momentum to induce fragmentation and subsequent binary formation. The magnetic field accumulates in the circumbinary disk where the removal and accretion of magnetic field are balanced, and finally drives the circumbinary outflow. This result explains the spectacular morphology of some specific young stellar objects such as L1551 IRS5. We can infer that most of the bipolar molecular outflows observed by low density tracers (i.e., CO) would correspond to circumbinary or circum-multiple outflows found in this Letter, since most of the young stellar objects are supposed to be binaries or multiples.

Language：English   Publishing type：Research paper (scientific journal)  

We examine emission from a young protostellar object (YPO) with three-dimensional ideal magnetohydrodynamic (MHD) simulations and three-dimensional non-local thermodynamic equilibrium line transfer calculations, and show the first results. To calculate the emission field, we employed a snapshot result of an MHD simulation having young bipolar outflows and a dense protostellar disk (a young circumstellar disk) embedded in an infalling envelope. Synthesized line emission of two molecular species (CO and SiO) shows that subthermally excited SiO lines as a high-density tracer can provide a better probe of the complex velocity field of a YPO, compared to fully thermalized CO lines. In a YPO at the earliest stage when the outflows are still embedded in the collapsing envelope, infall, rotation, and outflow motions have similar speeds. We find that the combined velocity field of these components introduces a great complexity in the line emissions through varying optical thickness and emissivity, such as asymmetric double-horn profiles. We show that the rotation of the outflows, one of the features that characterizes an outflow driven by magneto-centrifugal forces, appears clearly in velocity channel maps and intensity-weighted mean velocity (first moment of velocity) maps. The somewhat irregular morphology of the line emission at this youngest stage is dissimilar to a more evolved object such as young Class 0. High angular resolution observation by, e.g., the Atacama Large Millimeter/submillimeter Array telescope can reveal these features. Our results demonstrate a powerful potential of the synthesized emission of the three-dimensional line transfer to probe the velocity field embedded in the envelope, and further analysis will be able to determine the precise velocity field to assess the dynamics in the YPO to gain a better understanding of star formation.

Language：English   Publishing type：Research paper (scientific journal)  

We investigate the effects of viscosity on disk-planet interaction and discuss how type I migration of planets is modified. We have performed a linear calculation using shearing-sheet approximation and obtained the detailed, high-resolution density structure around the planet embedded in a viscous disk with a wide range of viscous coefficients. We use a time-dependent formalism that is useful in investigating the effects of various physical processes on disk-planet interaction. We find that the density structure in the vicinity of the planet is modified and the main contribution to the torque comes from this region, in contrast to the inviscid case. Although it is not possible to derive total torque acting on the planet within the shearing-sheet approximation, the one-sided torque can be very different from the inviscid case, depending on the Reynolds number. This effect has been neglected so far but our results indicate that the interaction between a viscous disk and a planet can be qualitatively different from an inviscid case and the details of the density structure in the vicinity of the planet are critically important.

Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)  

The standard scenario for the formation of planets has two critical problems: (1) Turbulence in the disk stirs up dust particles and prevents the dust sedimentation onto the disk mid-plane that is required for the fragmentation of the dusty layer prior to planetesimal formation. (2) The gravitational interaction of (proto)planets and the disk tends to result in their migration onto the central star within a short timescale. These problems have stimulated extensive theoretical work but still remain to be solved. In contrast, the recent increase of our understanding of the star formation process enables us to depict the long-term evolution of protoplanetary disks: the resultant gravitational fragmentation and the formation of gaseous planetary-mass objects in the disks. We critically review recent progress in our understanding of these processes and propose a possible hybrid scenario for the formation of planetary-mass objects in variety.

Language：English   Publishing type：Research paper (scientific journal)  

Brown dwarf formation and star formation efficiency are studied using a nested grid simulation that covers 5 orders of magnitude in spatial scale (104-0.1 AU). Starting with a rotating magnetized compact cloud with a mass of 0.22 M sun(225 M Jup), we follow the cloud evolution until the end of the main accretion phase. An outflow of ~5 km s-1 emerges ~100 yr before the protostar formation and does not disappear until the end of the calculation. The mass accretion rate declines from ~10-6 M sun yr-1 to ~10-8-10-12 M sun yr-1 in a short time (~104 yr) after the protostar formation. This is because (1) a large fraction of mass is ejected from the host cloud by the protostellar outflow and (2) the gas escapes from the host cloud by the thermal pressure. At the end of the calculation, 74% (167 M Jup) of the total mass (225 M Jup) is outflowing from the protostar, in which 34% (77 M Jup) of the total mass is ejected by the protostellar outflow with supersonic velocity and 40% (90 M Jup) escapes with subsonic velocity. On the other hand, 20% (45 M Jup) is converted into the protostar and 6% (13 M Jup) remains as the circumstellar disk. Thus, the star formation efficiency is epsilon = 0.2. The resultant protostellar mass is in the mass range of brown dwarfs. Our results indicate that brown dwarfs can be formed in compact cores in the same manner as hydrogen-burning stars, and the magnetic field and protostellar outflow are essential in determining the star formation efficiency and stellar mass.

Language：English   Publishing type：Research paper (scientific journal)  

We examine magnetohydrodynamic simulations of the propagation of a strong shock wave through the interstellar two-phase medium composed of small-scale cloudlets and diffuse warm neutral medium in two-dimensional geometry. The preshock two-phase medium is provided as a natural consequence of the thermal instability that is expected to be ubiquitous in the interstellar medium. We show that the shock-compressed shell becomes turbulent owing to the preshock density inhomogeneity, and magnetic field amplification takes place in the shell. The maximum field strength is determined by the condition that plasma β ~ 1, which gives the field strength on the order of 1 mG in the case of shock velocity ~103 km s-1. The strongly magnetized region shows filamentary and knotlike structures in two-dimensional simulations. The spatial scale of the regions with a magnetic field of ~1 mG in our simulation is roughly 0.05 pc, which is comparable to the spatial scale of the X-ray hot spots recently discovered in supernova remnants where the magnetic field strength is indicated to be amplified up to the order of 1 mG. This result may also suggest that the turbulent region with a locally strong magnetic field is expected to be spread out in the region with frequent supernova explosions, such as in the Galactic center and starburst galaxies.

Language：English   Publishing type：Research paper (scientific journal)  

We investigate the motion of a particle around a low-mass planet embedded in a nonturbulent gaseous disk. We take into account the effect of the gas structure that is modified by the gravitational interaction between the planets. We derive an analytic formula that describes the change of the semimajor axis of the particle due to the encounter with the planet using local approximation in a distant encounter regime. Our final formula includes the effects of steady, axisymmetric radial gas flow, the global gas pressure gradient in the disk, planet gravity, and the structure of the gas flow modified by the planet's gravity. We compare the analytic results with numerical calculations and indicate that our formula well describes the secular evolution of the dust particles' semimajor axes well, especially for small particles with large drag coefficients. We discuss the conditions for dust gap opening around a low-mass planet and radial distribution of dust particles. Our formula may provide a useful tool for calculating radial distribution of particles in a disk around the planet.

Language：English   Publishing type：Research paper (scientific journal)  

The radioactive isotope 60Fe (T 1/2 = 1.5 Myr) was present in the early solar system. It is unlikely that it was injected directly into the nascent solar system by a single, nearby supernova (SN). It is proposed instead that it was inherited during the molecular cloud (MC) stage from several SNe belonging to previous episodes of star formation. The expected abundance of 60Fe in star-forming regions is estimated taking into account the stochasticity of the star-forming process, and it is showed that many MCs are expected to contain 60Fe (and possibly 26Al [T 1/2 = 0.74 Myr]) at a level compatible with that of the nascent solar system. Therefore, no special explanation is needed to account for our solar system's formation.

Language：English   Publishing type：Research paper (scientific journal)  

The driving mechanisms of low- and high-velocity outflows in star formation processes are studied using three-dimensional resistive MHD simulations. Starting with a Bonnor-Ebert isothermal cloud rotating in a uniform magnetic field, we calculate cloud evolution from the molecular cloud core (nc=104 cm-3) to the stellar core (nc=1022 cm-3), where nc denotes the central density. In the collapsing cloud core, we found two distinct flows: low-velocity flows (~5 km s-1) with a wide opening angle, driven from the adiabatic core when the central density exceeds nc>~1012 cm-3; and high-velocity flows (~30 km s-1) with good collimation, driven from the protostar when the central density exceeds nc>~1021 cm-3. High-velocity flows are enclosed by low-velocity flows after protostar formation. The difference in the degree of collimation between the two flows is caused by the strength of the magnetic field and configuration of the magnetic field lines. The magnetic field around an adiabatic core is strong and has an hourglass configuration; therefore, flows from the adiabatic core are driven mainly by the magnetocentrifugal mechanism and guided by the hourglass-like field lines. In contrast, the magnetic field around the protostar is weak and has a straight configuration owing to ohmic dissipation in the high-density gas region. Therefore, flows from the protostar are driven mainly by the magnetic pressure gradient force and guided by straight field lines. Differing depth of the gravitational potential between the adiabatic core and the protostar causes the difference of flow speed. Low-velocity flows may correspond to the observed molecular outflows, while high-velocity flows may correspond to the observed optical jets. We suggest that the protostellar outflow and the jet are driven by different cores, rather than the outflow being entrained by the jet.

Language：English   Publishing type：Research paper (scientific journal)  

The saturation level of the magnetorotational instability (MRI) is investigated using three-dimensional MHD simulations. The shearing box approximation is adopted and the vertical component of gravity is ignored, so that the evolution of the MRI is followed in a small local part of the disk. We focus on the dependence of the saturation level of the stress on the gas pressure, which is a key assumption in the standard α disk model. From our numerical experiments we find that there is a weak power-law relation between the saturation level of the Maxwell stress and the gas pressure in the nonlinear regime; the higher the gas pressure, the larger the stress. Although the power-law index depends slightly on the initial field geometry, the relationship between stress and gas pressure is independent of the initial field strength and is unaffected by ohmic dissipation if the magnetic Reynolds number is at least 10. The relationship is the same in adiabatic calculations, where pressure increases over time, and nearly isothermal calculations, where pressure varies little with time. Over the entire region of parameter space explored, turbulence driven by the MRI has many characteristic ratios such as that of the Maxwell stress to the magnetic pressure. We also find that the amplitudes of the spatial fluctuations in density and the time variability in the stress are characterized by the ratio of magnetic pressure to gas pressure in the nonlinear regime. Our numerical results are qualitatively consistent with an idea that the saturation level of the MRI is determined by a balance between the growth of the MRI and the dissipation of the field through reconnection. The quantitative interpretation of the pressure-stress relation, however, may require advances in the theoretical understanding of nonsteady magnetic reconnection.

Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)  

Smoothed particle hydrodynamics is reformulated in terms of the convolution of the original hydrodynamics equations, and the new evolution equations for the particles are derived. The same evolution equation of motion is also derived using a new action principle. The force acting on each particle is determined by solving the Riemann problem. The use of the Riemann solver strengthens the method, making it accurate for the study of phenomena with strong shocks. The prescription for the variable smoothing length is shown. These techniques are implemented in strict conservation form. The results of a few test problems are also shown.

Language：English   Publishing type：Research paper (scientific journal)  

The propagation of a shock wave into an interstellar medium is investigated by two-dimensional numerical hydrodynamic calculation with cooling, heating, and thermal conduction. We present results of the high-resolution, two-dimensional calculations to follow the fragmentation that results from thermal instability in a shock-compressed layer. We find that the geometrically thin cooling layer behind the shock front fragments into small cloudlets. The cloudlets have supersonic velocity dispersion in the warm neutral medium, in which the fragments are embedded as cold condensations. The fragments tend to coalesce and become larger clouds.

Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)  

We carry out radiation hydrodynamic calculations to study physical processes in the formation of a 1 Msolar protostar. Following our previous work, calculations pursue the whole evolution from the beginning of the first collapse to the end of the main accretion phase. The adiabatic core formed after the initial collapse (i.e., the first core) experiences further gravitational collapse triggered by dissociation of molecular hydrogen, which leads to the formation of the second core, i.e., the birth of a protostar. The protostar grows in mass as accreting the infalling material from the circumstellar envelope, while the protostar keeps its radius at ~4 Rsolar during the main accretion phase. These typical features in the evolution are in good agreement with previous studies. We consider two different initial conditions for the density distribution: homogeneous and hydrostatic cloud cores with the same central density of 1.415x10-19 g cm-3 . The homogeneous core has the total mass of 1 Msolar while the hydrostatic core has 3.852 Msolar. For the initially homogeneous model, the accretion luminosity rapidly rises to the maximum value of 25 Lsolar just after the birth of a protostar, and declines gradually as the mass accretion rate decreases. In contrast, the luminosity increases monotonically with time for the initially hydrostatic model. This difference arises because the mass accretion rate varies depending on the inward acceleration at the initial stage, which affects the luminosity curve. A less massive hydrostatic core would possess the similar properties in the luminosity curve to the 3.852 Msolar case, because a hydrostatic cloud core with mass lower than 3.852 Msolar can be shown to provide a smaller mass accretion rate after the birth of a protostar and a more gradual rise in the luminosity curve. Our numerical code is designed to provide the evolution of the spectral energy distribution (SED) along with the dynamical evolution in our spherically symmetric calcu

Language：Japanese

Language：Japanese

Language：Japanese

わたしたちはいったいどこからきたのか．宇宙はどのようにしてはじまり今日に至ったのか．元素や夜空にきらめく星ぼしや，銀河誕生の謎に迫る．太陽系や地球，生命の誕生を経て人類に至る，美しく壮大な137億年の物語．ブラックホールや多様性も含め，研究の第一線に立つ11人が，最新情報から熱く語る．

Event date： 2023.3

Language：Japanese   Presentation type：Oral presentation (invited, special)  

Venue：Kyoto University   Country：Japan  

Event date： 2021.3

Language：English   Presentation type：Oral presentation (invited, special)  

Venue：Hiroshima, Japan   Country：Japan  

Event date： 2020.12

Language：English   Presentation type：Oral presentation (invited, special)  

Venue：Hong Kong   Country：Hong Kong  

Event date： 2020.12

Language：English   Presentation type：Oral presentation (invited, special)  

Venue：Gothenburg, Sweden   Country：Sweden  

Event date： 2020.5

Language：English   Presentation type：Oral presentation (invited, special)  

Venue：Zanjan, Iran   Country：Iran, Islamic Republic of  

Event date： 2009.11

Language：English   Presentation type：Oral presentation (invited, special)  

Recent development of the resistive MHD calculations of the gravitational collapse process from the
molecular cloud core is dramatical and enables us to understand early evolution of young stellar objects. I
will show further development on how MHD outflows affect on the formation of protoplanetary disks and
their evolutions. I will also discuss dynamical modelling of long-term evolution of magnetized disks and show
that the protoplanetary disks clear up inside-out by the disk wind driven by

Event date： 2009.8

Language：English   Presentation type：Oral presentation (invited, special)  

Event date： 2009.5

Language：English   Presentation type：Oral presentation (invited, special)  

Country：Japan