Shu-ichiro Inutsuka

ANGULAR MOMENTUM TRANSPORT BY MAGNETOHYDRODYNAMIC TURBULENCE IN ACCRETION DISKS: GAS PRESSURE DEPENDENCE OF THE SATURATION LEVEL OF THE MAGNETOROTATIONAL INSTABILITY

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.

A Radiation Hydrodynamic Model for Protostellar Collapse. I. The First Collapse

Dynamical collapse of a molecular cloud core and the formation of a star are investigated by per- forming radiation hydrodynamic calculations in spherical symmetry. The angle-dependent and frequency-dependent radiative transfer equation is solved without any di†usion approximations, and the evolution of the spectral energy distribution (SED) is examined. In the present paper, as the -rst step in a series of our work, evolutions before hydrogen molecules begin to dissociate (the so-called -rst collapse) are examined for di†erent masses and initial temperatures of the parent cloud cores and for di†erent opacities. Numerical results for a typical case K (T init \ 10 and K) D 0.01 cm2 g~1) show that the radius and mass of the -rst core are D 5A U and D0.05 i P (10 respectively. These values are independent both of the mass of the parent cloud core and of the M _ , initial density pro-le. The analytical expressions for the radius, mass, and accretion luminosity of the -rst core are also obtained. The SED contains only cold components of a few times 10 K throughout the -rst collapse phase, because the opaque envelope veils the -rst core from observers. We suggest that the molecular cloud cores with luminosities higher than D0.1 should contain young protostars deep L _ in the center, even if they show no evidence for the existence of central stars in near-infrared and optical observations. Subject headings: hydrodynamics E ISM: clouds E methods: numerical E radiative transfer E stars: formation

Self-similar solutions and the stability of collapsing isothermal filaments

In this paper self-similar solutions which describe collapsing isothermal cylinders with self-gravity are derived. The solutions are parameterized by their line masses. Their stability is also investigated by two different methods in the linear regime. One is the approximate separation of variables as an eigenvalue problem and the other is direct numerical integration of the evolution of perturbations.

Molecular Cloud Formation in Shock-compressed Layers

We investigate the propagation of a shock wave into a warm neutral medium and a cold neutral medium by one-dimensional hydrodynamic calculations with detailed treatment of thermal and chemical processes. Our main result shows that thermal instability inside the shock-compressed layer produces a geometrically thin, dense layer in which a large amount of hydrogen molecules are formed. Linear stability analysis suggests that this thermally collapsed layer will fragment into small molecular cloudlets. We expect that frequent compression due to supernova explosions, stellar winds, spiral density waves, etc., in the Galaxy causes the interstellar medium to be occupied by these small molecular cloudlets.

An origin of supersonic motions in interstellar clouds

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.

Molecular Evolution in Collapsing Prestellar Cores

We have investigated the evolution and distribution of molecules in collapsing prestellar cores via numerical chemical models, adopting the Larson-Penston solution and its delayed analogs to study collapse. Molecular abundances and distributions in a collapsing core are determined by the balance among the dynamical, chemical, and adsorption timescales. When the central density nH of a prestellar core with the Larson-Penston flow rises to 3 × 106 cm-3, the CCS and CO column densities are calculated to show central holes of radius 7000 and 4000 AU, respectively, while the column density of N2H+ is centrally peaked. These predictions are consistent with observations of L1544. If the dynamical timescale of the core is larger than that of the Larson-Penston solution owing to magnetic fields, rotation, or turbulence, the column densities of CO and CCS are smaller, and their holes are larger than in the Larson-Penston core with the same central gas density. On the other hand, N2H+ and NH3 are more abundant in the more slowly collapsing core. Therefore, molecular distributions can probe the collapse timescale of prestellar cores. Deuterium fractionation has also been studied via numerical calculations. The deuterium fraction in molecules increases as a core evolves and molecular depletion onto grains proceeds. When the central density of the core is nH = 3 × 106 cm-3, the ratio DCO+/HCO+ at the center is in the range 0.06-0.27, depending on the collapse timescale and adsorption energy; this range is in reasonable agreement with the observed value in L1544.

A Production Mechanism for Clusters of Dense Cores

Collapse and fragmentation processes within filamentary interstellar molecular clouds are investigated in detail. A quasi-equilibrium filament fragments into dense cores separated by about 4 times the filament diameter. Nonlinear calculations reveal that the central region of each core tends toward spherical collapse and further hierarchical fragmentation is not expected. However, merging and clustering of cores tend to occur soon after the fragmentation. When the line mass of an isothermal filament exceeds the critical value for equilibrium by a small amount, perturbations do not grow much, and the entire filament collapses toward the axis without fragmenting. In this case no characteristic scale for fragmentation appears during the isothermal collapse phase. Subsequent evolution is also investigated. A change of the equation of state yields a characteristic density, separation length, and mass for fragmentation. These values correspond to 5 × 10-15 g cm-3, 2 × 10-3 pc, and 4 × 10-2 M☉, if the cloud temperature is 10 K. These results are consistent with recent high-resolution radio observations of dense cores in Taurus dark cloud.

HIGH-AND LOW-VELOCITY MAGNETIZED OUTFLOWS IN THE STAR FORMATION PROCESS IN A GRAVITATIONALLY COLLAPSING CLOUD

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.

Direct Imaging of Fine Structures in Giant Planet Forming Regions of the Protoplanetary Disk around AB Aurigae

We report high-resolution 1.6 μm polarized intensity (PI) images of the circumstellar disk around the Herbig Ae star AB Aur at a radial distance of 22 AU (015) up to 554 AU (385), which have been obtained by the high-contrast instrument HiCIAO with the dual-beam polarimetry. We revealed complicated and asymmetrical structures in the inner part (140 AU) of the disk while confirming the previously reported outer (r 200 AU) spiral structure. We have imaged a double ring structure at ~40 and ~100 AU and a ring-like gap between the two. We found a significant discrepancy of inclination angles between two rings, which may indicate that the disk of AB Aur is warped. Furthermore, we found seven dips (the typical size is ~45 AU or less) within two rings, as well as three prominent PI peaks at ~40 AU. The observed structures, including a bumpy double ring, a ring-like gap, and a warped disk in the innermost regions, provide essential information for understanding the formation mechanism of recently detected wide-orbit (r > 20 AU) planets.

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

Using three-dimensional magnetohydrodynamic simulations, we investigate general properties of a blast wave shock interacting with interstellar clouds. The pre-shock cloudy medium is generated as a natural consequence of the thermal instability that simulates realistic clumpy interstellar clouds and their diffuse surrounding. The shock wave that sweeps the cloudy medium generates a turbulent shell through the vorticity generations that are induced by shock-cloud interactions. In the turbulent shell, the magnetic field is amplified as a result of turbulent dynamo action. The energy density of the amplified magnetic field can locally grow comparable to the thermal energy density, particularly at the transition layers between clouds and the diffuse surrounding. In the case of a young supernova remnant (SNR) with a shock velocity 103?km?s?1, the corresponding strength of the magnetic field is approximately 1?mG. The propagation speed of the shock wave is significantly stalled in the clouds because of the high density, while the shock maintains a high velocity in the diffuse surrounding. In addition, when the shock wave hits the clouds, reflection shock waves are generated that propagate back into the shocked shell. From these simulation results, many observational characteristics of the young SNR RX?J1713.7?3946 that is suggested to be interacting with molecular clouds can be explained as follows. The reflection shocks can accelerate particles in the turbulent downstream region where the magnetic field strength reaches 1?mG, which causes short-time variability of synchrotron X-rays. Since the shock velocity is stalled locally in the clouds, the temperature in the shocked cloud is suppressed far below 1 keV. Thus, thermal X-ray line emission would be faint even if the SNR is interacting with molecular clouds. We also find that the photon index of the ?0-decay gamma rays generated by cosmic-ray protons can be 1.5 (corresponding energy flux is ?F ??0.5) because the penetration depth of high-energy particles into the clumpy clouds depends on their energy. This suggests that, if we rely only on the spectral study, the hadronic gamma-ray emission is indistinguishable from the leptonic inverse Compton emission. We propose that the spatial correlation of the gamma-ray, X-ray, and CO line-emission regions can be conclusively used to understand the origin of gamma rays from RX?J1713.7?3946.