Faculty Profiles - INUTSUKA, Shu-ichiro

Papers - INUTSUKA, Shu-ichiro

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THE NONLINEAR OHM'S LAW: PLASMA HEATING BY STRONG ELECTRIC FIELDS AND ITS EFFECTS ON THE IONIZATION BALANCE IN PROTOPLANETARY DISKS Reviewed Open Access

Okuzumi Satoshi, Inutsuka Shu-ichiro

ASTROPHYSICAL JOURNAL Vol. 800 ( 1 ) 2015.2

Effects of radiative transfer on the structure of self-gravitating discs, their fragmentation and the evolution of the fragments Reviewed Open Access

Tsukamoto, Y.; Takahashi, S. Z.; Machida, M. N.; Inutsuka, S.

Monthly Notices of the Royal Astronomical Society Vol. 446 ( 2 ) page： 1175-1190 2015.1

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

Two-component Secular Gravitational Instability in a Protoplanetary Disk: A Possible Mechanism for Creating Ring-like Structures Reviewed Open Access

Takahashi, Sanemichi Z.; Inutsuka, Shu-ichiro

The Astrophysical Journal Vol. 794 ( 1 ) page： 55 (7) 2014.10

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

A new scheme of causal viscous hydrodynamics for relativistic heavy-ion collisions: A Riemann solver for quark-gluon plasma Reviewed Open Access

Akamatsu, Yukinao; Inutsuka, Shu-ichiro; Nonaka, Chiho; Takamoto, Makoto

Journal of Computational Physics Vol. 256 ( 1 ) page： 34-54 2014.1

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

Crowding-out of Giants by Dwarfs: An Origin for the Lack of Companion Planets in Hot Jupiter Systems Reviewed Open Access

Ogihara, Masahiro; Inutsuka, Shu-ichiro; Kobayashi, Hiroshi

The Astrophysical Journal Letters Vol. 778 ( 1 ) page： L9 2013.11

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

Present-day star formation: From molecular cloud cores to protostars and protoplanetary disks Invited Reviewed Open Access

Shu-ichiro Inutsuka

Progress of Theoretical and Experimental Physics Vol. 2012 ( 1 ) page： id.01A307, 25 2012.10

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

TOWARD UNDERSTANDING THE ORIGIN OF TURBULENCE IN MOLECULAR CLOUDS: SMALL-SCALE STRUCTURES AS UNITS OF DYNAMICAL MULTI-PHASE INTERSTELLAR MEDIUM Reviewed Open Access

Kengo Tachihara, Kazuya Saigo, Aya E. Higuchi, Tsuyohshi Inoue, Shu-ichiro Inutsuka, Moritz Hackstein, Martin Haas, Markus Mugrauer

ASTROPHYSICAL JOURNAL Vol. 754 ( 2 ) 2012.8

TOWARD UNDERSTANDING THE COSMIC-RAY ACCELERATION AT YOUNG SUPERNOVA REMNANTS INTERACTING WITH INTERSTELLAR CLOUDS: POSSIBLE APPLICATIONS TO RX J1713.7-3946 Open Access

Tsuyoshi Inoue, Ryo Yamazaki, Shu-ichiro Inutsuka, Yasuo Fukui

ASTROPHYSICAL JOURNAL Vol. 744 ( 1 ) 2012.1

Gravitational Fragmentation of Expanding Shells: I. Linear Analysis Reviewed Open Access

K. Iwasaki, S. Inutsuka, & T. Tsuribe

The Astrophysical Journal Vol. 733 ( 1 ) page： 16 (12pp) 2011.5

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

Gravitational Fragmentation of Expanding Shells. II. Three-dimensional Simulations Reviewed Open Access

K. Iwasaki, S. Inutsuka, & T. Tsuribe

The Astrophysical Journal Vol. 733 ( 1 ) page： 17 (11) 2011.5

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

Recurrent Planet Formation and Intermittent Protostellar Outflows Induced by Episodic Mass Accretion Reviewed

Masahiro Machida, Shu-ichiro Inutsuka, & Tomoaki Matsumoto

The Astrophysical Journal Vol. in press 2011.1

Formation Process of the Circumstellar Disk: Long-term Simulations in the Main Accretion Phase of Star Formation Reviewed

Masahiro Machida, Shu-ichiro Inutsuka, & Tomoaki Matsumoto

The Astrophysical Journal Vol. 724 ( 2 ) page： 1006 2010.12

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.

TWO-DIMENSIONAL STUDY OF THE PROPAGATION OF PLANETARY WAKE AND THE INDICATION OF GAP OPENING IN AN INVISCID PROTOPLANETARY DISK Open Access

Takayuki Muto, Takeru K. Suzuki, Shu-ichiro Inutsuka

ASTROPHYSICAL JOURNAL Vol. 724 ( 1 ) page： 448 - 463 2010.11

TWO-STEP ACCELERATION MODEL OF COSMIC RAYS AT MIDDLE-AGED SUPERNOVA REMNANTS: UNIVERSALITY IN SECONDARY SHOCKS Open Access

Tsuyoshi Inoue, Ryo Yamazaki, Shu-ichiro Inutsuka

ASTROPHYSICAL JOURNAL LETTERS Vol. 723 ( 1 ) page： L108 - L112 2010.11

The relativistic kinetic dispersion relation: Comparison of the relativistic Bhatnagar-Gross-Krook model and Grad's 14-moment expansion Reviewed Open Access

Makoto Takamoto & Shu-ichiro Inutsuka

Physica Vol. 389 ( 21 ) page： 4580 2010.11

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

Two-Dimensional Study of the Propagation of Planetary Wake and the Indication to Gap Opening in an Inviscid Protoplanetary Disk Reviewed

Takayuki Muto, Takeru K. Suzuki, & Shu-ichiro Inutsuka

The Astrophysical Journal Vol. 724 page： 448 2010.11

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.

Two-Step Acceleration Model of Cosmic Rays at Middle-Aged Supernova Remnants: Universality in Secondary Shocks Reviewed

Tsuyoshi Inoue, Ryo Yamazaki, & Shu-ichiro Inutsuka

The Astrophysical Journal Letters Vol. 723 ( 1 ) page： L108 2010.11

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.

Protoplanetary Disk Winds via Magnetorotational Instability: Formation of an Inner Hole and a Crucial Assist for Planet Formation Reviewed

Takeru K. Suzuki, Takayuki Muto, & Shu-ichiro Inutsuka

The Astrophysical Journal Vol. 718 ( 2 ) page： 1289-1304 2010.8

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.

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

Shugo Michikoshi, Eiichiro Kokubo, & Shu-ichiro Inutsuka

The Astrophysical Journal Vol. 719 page： 1021 2010.8

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.

Emergence of Protoplanetary Disks and Successive Formation of Gaseous Planets by Gravitational Instability Reviewed

Shu-ichiro Inutsuka, Masahiro Machida, & Tomoaki Matsumoto

The Astrophysical Journal Letters Vol. 718 page： L58 2010.8

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.

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Papers - INUTSUKA, Shu-ichiro