研究者詳細 - 犬塚　修一郎

論文 - 犬塚　修一郎

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N-Body Simulation of Planetesimal Formation through Gravitational Instability of a Dust Layer in Laminar Gas Disk 査読有り

Shugo Michikoshi, Eiichiro Kokubo, & Shu-ichiro Inutsuka

The Astrophysical Journal 719 巻頁： 1021 2010年8月

記述言語：英語掲載種別：研究論文（学術雑誌）

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.

Emergence of Protoplanetary Disks and Successive Formation of Gaseous Planets by Gravitational Instability 査読有り

Shu-ichiro Inutsuka, Masahiro Machida, & Tomoaki Matsumoto

The Astrophysical Journal Letters 718 巻頁： L58 2010年8月

記述言語：英語掲載種別：研究論文（学術雑誌）

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.

Gas accretion onto a protoplanet and formation of a gas giant planet 査読有り

Masahiro Machida, Eiichiro Kokubo, Shu-ichiro Inutsuka, & Tomoaki Matsumoto

Monthly Notices of the Royal Astronomical Society 405 巻頁： 1227 2010年6月

記述言語：英語掲載種別：研究論文（学術雑誌）

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.

Analysis of the Relaxation Process using Non-Relativistic Kinetic Equation 査読有り

Progress of Theoretical Physics

Progress of Theoretical Physics 123 巻頁： 903 2010年5月

記述言語：英語掲載種別：研究論文（学術雑誌）

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.

Kelvin-Helmholtz instabilities with Godunov smoothed particle hydrodynamics 査読有り

Seung-Hoon Cha, Shu-ichiro Inutsuka, & Sergei Nayakshin

Monthly Notices of the Royal Astronomical Society 9999 巻頁： 1-10 2010年1月

記述言語：英語掲載種別：研究論文（学術雑誌）

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.

Binary formation with different metallicities: dependence on initial conditions 査読有り

Masahiro N. Machida, Kazuyuki Omukai, Tomoaki Matsumoto, & Shu-ichiro Inutsuka

Monthly Notices of the Royal Astronomical Society 399 巻 ( 3 ) 頁： 1255-1263 2009年11月

記述言語：英語掲載種別：研究論文（学術雑誌）

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.

N-body Simulation of Planetesimal Formation Through Gravitational Instability and Coagulation. II. Accretion Model 査読有り

Shugo Michikoshi, Eiichiro Kokubo, & Shu-ichiro Inutsuka

The Astrophysical Journal 703 巻 ( 2 ) 頁： 1363-1373 2009年10月

記述言語：英語掲載種別：研究論文（学術雑誌）

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.

Two-Fluid Magnetohydrodynamics Simulations of Converging H I Flows in the Interstellar Medium. II. Are Molecular Clouds Generated Directly from a Warm Neutral Medium? 査読有り

Tsuyoshi Inoue & Shu-ichiro Inutsuka

The Astrophysical Journal 704 巻 ( 1 ) 頁： 161-169 2009年10月

記述言語：英語掲載種別：研究論文（学術雑誌）

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.

The Circumbinary Outflow: A Protostellar Outflow Driven by a Circumbinary Disk 査読有り

Masahiro N. Machida, Shu-ichiro Inutsuka, & Tomoaki Matsumoto

The Astrophysical Journal 704 巻 ( 1 ) 頁： L10-L14 2009年10月

記述言語：英語掲載種別：研究論文（学術雑誌）

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.

Emission from a Young Protostellar Object I. Signatures of Young Embedded Outflows 査読有り

Masako Yamada, Masahiro N. Machida, Shu-ichiro Inutsuka, & Kohji Tomisaka

The Astrophysical Journal 703 巻 ( 1 ) 頁： 1141-1158 2009年9月

記述言語：英語掲載種別：研究論文（学術雑誌）

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.

A Hybrid Scenario for Planet Formation

Shu-ichiro Inutsuka

AIP Conference Proceedings 1158 巻頁： 31-34 2009年8月

担当区分：筆頭著者記述言語：英語掲載種別：研究論文（学術雑誌）

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.

Local Linear Analysis of Interaction between a Planet and Viscous Disk and an Implication on Type I Planetary Migration 査読有り

Takayuki Muto & Shu-ichiro Inutsuka

The Astrophysical Journal 701 巻 ( 1 ) 頁： 18-31 2009年8月

記述言語：英語掲載種別：研究論文（学術雑誌）

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.

First Direct Simulation of Brown Dwarf Formation in a Compact Cloud Core 査読有り

Masahiro N. Machida, Shu-ichiro Inutsuka, & Tomoaki Matsumoto

The Astrophysical Journal 699 巻 ( 2 ) 頁： L157 2009年7月

記述言語：英語掲載種別：研究論文（学術雑誌）

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.

Turbulence and Magnetic Field Amplification in Supernova Remnants: Interactions Between a Strong Shock Wave and Multiphase Interstellar Medium 査読有り

Tsuyoshi Inoue, Ryo Yamazaki, & Shu-ichiro Inutsuka

The Astrophysical Journal 695 巻 ( 2 ) 頁： 825-833 2009年4月

記述言語：英語掲載種別：研究論文（学術雑誌）

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.

Orbital Evolution of a Particle Interacting with a Single Planet in a Protoplanetary Disk 査読有り

Takayuki Muto & Shu-ichiro Inutsuka

The Astrophysical Journal 695 巻 ( 2 ) 頁： 1132-1150 2009年4月

記述言語：英語掲載種別：研究論文（学術雑誌）

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.

Supernova Propagation and Cloud Enrichment: A New Model for 60Fe Origin in Early Solar System 査読有り

Matthieu Gounelle, Anders Meibom, Patrick Hennebelle & Shu-ichiro Inutsuka

The Astrophysical Journal 694 巻 ( 1 ) 頁： L1 2009年3月

記述言語：英語掲載種別：研究論文（学術雑誌）

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.

*重力収縮する雲の中での星形成過程における高速と低速の磁気天体風について査読有り

町田正博，犬塚修一郎，松本倫明

米国天体物理学会誌 676 巻 ( 1 ) 頁： 1088-1108 2008年4月

*降着円盤における磁気流体力学的乱流による角運動量輸送：磁気回転不安定性のガス圧依存性査読有り

佐野孝好，犬塚修一郎，ニール・ターナー，ジェームズ・ストーン

米国天文学会誌 605 巻 ( 1 ) 頁： 321-339 2004年4月

*リーマン問題解法を用いたＳＰＨ法の再定式化査読有り

犬塚修一郎

(米国)計算物理学ジャーナル 179 巻 ( 1 ) 頁： 238-267 2002年6月

担当区分：筆頭著者記述言語：英語掲載種別：研究論文（学術雑誌）

流体力学の方程式の畳込みを用いてSPH法は再定式化され，新しい時間発展の方程式は導出される．その式は最小作用の原理を用いても得られる．粒子にかかる力はリーマン問題解法を用いて決められるため，衝撃波を含む現象においても精度の良い記述が可能となる．可変スムージング長の場合の処方箋も示す．この計算法は完全に保存形式を保つように構築されている．いくつかのテスト問題も示される．

*星間雲における超音速運動の起源査読有り

小山洋，犬塚修一郎

米国天体物理学会誌 564 巻 ( 2 ) 頁： L97-L100 2002年1月

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