arXiv subjects

Kohji Tomisaka

Publications and source records attributed to Kohji Tomisaka.

At least 73 records · Page 4Linked to original sources

arXiv2010

Radiation Magnetohydrodynamics Simulation of Proto-Stellar Collapse: Two-Component Molecular Outflow

We perform a three-dimensional nested-grid radiation magneto-hydrodynamics (RMHD) simulation with self-gravity to study the early phase of the low-mass star formation process from a rotating molecular cloud core to a first adiabatic core just before the second collapse begins. Radiation transfer is handled with the flux-limited diffusion approximation, operator-splitting and implicit time-integrator. In the RMHD simulation, the outer region of the first core attains a higher entropy and the size of first core is larger than that in the magnetohydrodynamics simulations with the barotropic approximation. Bipolar molecular outflow consisting of two components is driven by magnetic Lorentz force via different mechanisms, and shock heating by the outflow is observed. Using the RMHD simulation we can predict and interpret the observed properties of star-forming clouds, first cores and outflows with millimeter/submillimeter radio interferometers, especially the Atacama Large Millimeter/submillimeter Array (ALMA).

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arXiv2009

Formation of Galactic Center Magnetic Loops

A survey for the molecular clouds in the Galaxy with NANTEN mm telescope has discovered molecular loops in the Galactic center region. The loops show monotonic gradients of the line of sight velocity along the loops and the large velocity dispersions towards their foot points. It is suggested that these loops are explained in terms of the buoyant rise of magnetic loops due to the Parker instability. We have carried out global three-dimensional magneto-hydrodynamic simulations of the gas disk in the Galactic center. The gravitational potential is approximated by the axisymmetric potential proposed by Miyamoto & Nagai (1975). At the initial state, we assume a warm (~ 10^4 K) gas torus threaded by azimuthal magnetic fields. Self-gravity and radiative cooling of the gas are ignored. We found that buoyantly rising magnetic loops are formed above the differentially rotating, magnetically turbulent disk. By analyzing the results of global MHD simulations, we have identified individual loops, about 180 in the upper half of the disk, and studied their statistical properties such as their length, width, height, and velocity distributions along the loops. Typical length and height of a loop are 1kpc and 200pc, respectively. The line of sight velocity changes linearly along a loop and shows large dispersions around the foot-points. Numerical results indicate that loops emerge preferentially from the region where magnetic pressure is large. We argue that these properties are consistent with those of the molecular loops discovered by NANTEN.

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arXiv2009

Toward First-Principle Simulations of Galaxy Formation: II. Shock-Induced Starburst at a Collision Interface During the First Encounter of Interacting Galaxies

We investigated the evolution of interacting disk galaxies using high-resolution $N$-body/SPH simulations, taking into account the multiphase nature of the interstellar medium (ISM). In our high-resolution simulations, a large-scale starburst occurred naturally at the collision interface between two gas disks at the first encounter, resulting in the formation of star clusters. This is consistent with observations of interacting galaxies. The probability distribution function (PDF) of gas density showed clear change during the galaxy-galaxy encounter. The compression of gas at the collision interface between the gas disks first appears as an excess at $n_{\rm H} \sim 10{\rm cm^{-3}}$ in the PDF, and then the excess moves to higher densities ($n_{\rm H} \gtrsim 100{\rm cm^{-3}}$) in a few times $10^7$ years where starburst takes place. After the starburst, the PDF goes back to the quasi-steady state. These results give a simple picture of starburst phenomena in galaxy-galaxy encounters.

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arXiv2008

Three Dimensional Line Transfer Study of Energy Sources in Compact Molecular Gas in Active Galaxies -- AGN/Starburst Connection --

Recent observations suggest molecular line ratios in millimeter and submillimeter bands may be a good tool to reveal the long-standing question on the origin of energy sources in obscured active galaxies -- AGN and/or starburst. Observations of actual molecular medium show in general inhomogeneous structures as well as high-resolution hydrodynamic simulations do. In order for precise interpretation of emergent line emission from the inhomogeneous molecular gas to probe the dominant energy source of active galaxies, we study characteristic features of emergent intensities via three-dimensional non-LTE (non-local thermodynamic equilibrium) line transfer simulations. Our results succeeded in making clear 1) the necessary conditions for HCN/HCO$^{+}$-dichotomy, and 2) importance of clumpiness on intensity ratio and its interpretation. These results are obtained for the first time by our realistic three-dimensional simulations, and line transfer simulations will be a powerful tool to comprehensive studies of extragalactic interstellar medium (ISM) in forthcoming ALMA (Atacama Large Millimeter/submillimeter Array) era.

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arXiv2008

How was the mushroom-shaped GW 123.4--1.5 formed in the Galactic disk?

The unusual mushroom-shaped HI cloud, GW 123.4--1.5, is hundreds of parsecs in size but does not show any correlations to HI shells or chimney structures. To investigate the origin and velocity structure of GW 123.4--1.5, we perform three-dimensional hydrodynamical simulations of the collision of a high-velocity cloud with the Galactic disk. We also perform a parameter study of the density, radius, and incident angle of the impact cloud. The numerical experiments indicate that we reproduce the mushroom-shaped structure which resembles GW 123.4--1.5 in shape, size, position-velocity across the cap of the mushroom, and the density ratio between the mushroom and surrounding gas. GW 123.4--1.5 is expected to be formed by the almost head-on collision of a HVC with velocity $\sim 100 \kms$ and mass $\sim 10^5 \Msun$ about $5 \times 10^7 \yr$ ago. A mushroom-shaped structure like GW 123.4--1.5 must be infrequent on the Galactic plane, because the head-on collision which explains the mushroom structure seems rare for observed HVCs. HVC-disk collision explains not only the origin of the mushroom-shaped structure but also the formation of a variety of structures like shells, loops, and vertical structures in our Galaxy.

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arXiv2008

Toward first-principle simulations of galaxy formation: I. How should we choose star formation criteria in high-resolution simulations of disk galaxies?

We performed 3-dimensional N-body/SPH simulations to study how mass resolution and other model parameters such as the star formation efficiency parameter, C* and the threshold density, nth affect structures of the galactic gaseous/stellar disk in a static galactic potential. We employ 10^6 - 10^7 particles to resolve a cold and dense (T < 100 K & n_H > 100 cm^{-3}) phase. We found that structures of the ISM and the distribution of young stars are sensitive to the assumed nth. High-nth models with nth = 100 cm^{-3} yield clumpy multi-phase features in the ISM. Young stars are distributed in a thin disk of which half-mass scale height is 10 - 30 pc. In low-nth models with nth = 0.1 cm^{-3}, the stellar disk is found to be several times thicker, and the gas disk appears smoother than the high-nth models. A high-resolution simulation with high-nth is necessary to reproduce the complex structure of the gas disk. The global properties of the model galaxies in low-nth models, such as star formation histories, are similar to those in the high-nth models when we tune the value of C* so that they reproduce the observed relation between surface gas density and surface star formation rate density. We however emphasize that high-nth models automatically reproduce the relation, regardless of the values of C*. The ISM structure, phase distribution, and distributions of young star forming region are quite similar between two runs with values of C* which differ by a factor of 15. We also found that the timescale of the flow from n_H ~1 cm^{-3} to n_H > 100 cm^{-3} is about 5 times as long as the local dynamical time and is independent of the value of C*. The use of a high-nth criterion for star formation in high-resolution simulations makes numerical models fairy insensitive to the modelling of star formation. (Abridged)

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arXiv2007

Formation Scenario for Wide and Close Binary Systems

Fragmentation and binary formation processes are studied using three-dimensional resistive MHD nested grid simulations. Starting with a Bonnor-Ebert isothermal cloud rotating in a uniform magnetic field, we calculate the cloud evolution from the molecular cloud core (n=10^4 cm^-3) to the stellar core (n \simeq 10^22 cm^-3). We calculated 147 models with different initial magnetic, rotational, and thermal energies, and the amplitudes of the non-axisymmetric perturbation. In a collapsing cloud, fragmentation is mainly controlled by the initial ratio of the rotational to the magnetic energy, regardless of the initial thermal energy and amplitude of the non-axisymmetric perturbation. When the clouds have large rotational energies in relation to magnetic energies, fragmentation occurs in the low-density evolution phase (10^12 cm^-3 < n < 10^15 cm^-3) with separations of 3-300 AU. Fragments that appeared in this phase are expected to evolve into wide binary systems. On the other hand, fragmentation does not occur in the low-density evolution phase, when initial clouds have large magnetic energies in relation to the rotational energies. In these clouds, fragmentation only occurs in the high-density evolution phase (n > 10^17 cm^-3) after the clouds experience significant reduction of the magnetic field owing to Ohmic dissipation in the period of 10^12 cm^-3 < n < 10^15 cm^-3. Fragments appearing in this phase have separations of < 0.3 AU, and are expected to evolve into close binary systems. As a result, we found two typical fragmentation epochs, which cause different stellar separations. Although these typical separations are disturbed in the subsequent gas accretion phase, we might be able to observe two peaks of binary separations in extremely young stellar groups.

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arXiv2007

HCN to HCO^+ Millimeter Line Diagnostics of AGN Molecular Torus I : Radiative Transfer Modeling

We explore millimeter line diagnostics of an obscuring molecular torus modeled by a hydrodynamic simulation with three-dimensional nonLTE radiative transfer calculations. Based on the results of high-resolution hydrodynamic simulation of the molecular torus around an AGN, we calculate intensities of HCN and HCO^{+} rotational lines as two representative high density tracers. The three-dimensional radiative transfer calculations shed light on a complicated excitation state in the inhomogeneous torus, even though a spatially uniform chemical structure is assumed. Our results suggest that HCN must be much more abundant than HCO^{+} in order to obtain a high ratio ($R_{HCN/HCO+}\sim 2$) observed in some of the nearby galaxies. There is a remarkable dispersion in the relation between integrated intensity and column density, indicative of possible shortcomings of HCN(1-0) and HCO^{+}(1-0) lines as high density tracers. The internal structures of the inhomogeneous molecular torus down to subparsec scale in external galaxies will be revealed by the forthcoming Atacama Large Millimeter/submillimeter Array (ALMA). The three-dimensional radiative transfer calculations of molecular lines with high-resolution hydrodynamic simulation prove to be a powerful tool to provide a physical basis for molecular line diagnostics of the central regions of external galaxies.

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arXiv2006

Current Advances in the Methodology and Computational Simulation of the Formation of Low-Mass Stars

Developing a theory of low-mass star formation ($\sim 0.1$ to 3~M$_{\odot}$) remains one of the most elusive and important goals of theoretical astrophysics. The star-formation process is the outcome of the complex dynamics of interstellar gas involving non-linear interactions of turbulence, gravity, magnetic field and radiation. The evolution of protostellar condensations, from the moment they are assembled by turbulent flows to the time they reach stellar densities, spans an enormous range of scales, resulting in a major computational challenge for simulations. Since the previous Protostars and Planets conference, dramatic advances in the development of new numerical algorithmic techniques have been successfully implemented on large scale parallel supercomputers. Among such techniques, Adaptive Mesh Refinement and Smooth Particle Hydrodynamics have provided frameworks to simulate the process of low-mass star formation with a very large dynamic range. It is now feasible to explore the turbulent fragmentation of molecular clouds and the gravitational collapse of cores into stars self-consistently within the same calculation. The increased sophistication of these powerful methods comes with substantial caveats associated with the use of the techniques and the interpretation of the numerical results. In this review, we examine what has been accomplished in the field and present a critique of both numerical methods and scientific results. We stress that computational simulations should obey the available observational constraints and demonstrate numerical convergence. Failing this, results of numerical simulations do not advance our understanding of low-mass star formation.

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arXiv2006

Evolution of Rotating Molecular Cloud Core with Oblique Magnetic Field

We studied the collapse of rotating molecular cloud cores with inclined magnetic fields, based on three-dimensional numerical simulations.The numerical simulations start from a rotating Bonnor-Ebert isothermal cloud in a uniform magnetic field. The magnetic field is initially taken to be inclined from the rotation axis. As the cloud collapses, the magnetic field and rotation axis change their directions. When the rotation is slow and the magnetic field is relatively strong, the direction of the rotation axis changes to align with the magnetic field, as shown earlier by Matsumoto & Tomisaka. When the magnetic field is weak and the rotation is relatively fast, the magnetic field inclines to become perpendicular to the rotation axis. In other words, the evolution of the magnetic field and rotation axis depends on the relative strength of the rotation and magnetic field. Magnetic braking acts to align the rotation axis and magnetic field, while the rotation causes the magnetic field to incline through dynamo action. The latter effect dominates the former when the ratio of the angular velocity to the magnetic field is larger than a critical value Ω_0/ B_0 > 0.39 G^1/2 c_s^-1, where B_0, Ω_0, G, and c_s^-1 denote the initial magnetic field, initial angular velocity, gravitational constant, and sound speed, respectively. When the rotation is relatively strong, the collapsing cloud forms a disk perpendicular to the rotation axis and the magnetic field becomes nearly parallel to the disk surface in the high density region. A spiral structure appears due to the rotation and the wound-up magnetic field in the disk.

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arXiv2005

Alignment of Outflows with Magnetic Fields in Cloud Cores

We estimate the polarized thermal dust emission from MHD simulations of protostellar collapse and outflow formation in order to investigate alignment of outflows with magnetic fields. The polarization maps indicate that alignment of an outflow with the magnetic field depends on the field strength inside the cloud core; the direction of the outflow, projected on the plane of the sky, is aligned preferentially with the mean polarization vector for a cloud core with a magnetic field strength of 80 microgauss, while it does not tend to be aligned for 50 microgauss as long as the 1000 AU scale is considered. The direction of the magnetic field at the cloud center is probed by the direction of the outflow. In addition, the magnetic field at the cloud center can be revealed by ALMA even when the source is embedded deeply in the envelope. The Chandrasekhar-Fermi formula is examined using the polarization maps, indicating that the field strength predicted by the formula should be corrected by a factor of 0.24 - 0.44. The correction factor has a tendency to be lower for a cloud core with a weaker magnetic field.

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arXiv2005

The Evolution of the First Core in Rotating Molecular Core

We present the evolution path of the first core in rotating molecular cloud core and the evolution during the second collapse phase using axisymmetric numerical calculations. The structure and evolution of the rotating first core is characterized by the angular momentum $J_{\rm core}$ and mass $M_{\rm core}$, both of which increase with time by accretion from the infalling envelope. We find the evolution path of it can be considered as a sequence of equilibrium solutions with a constant $J_{\rm core}/M_{\rm core}^2$. Such evolution paths are in good agreement with the results of three-dimensional numerical simulations. The evolution paths is completely different from spherical case. Except for extremely small rotation rate of $J_{\rm core}/M_{\rm core}^2 < 0.015 G/(\sqrt{2} c_{\rm iso})$, the first core does not increase in the central density beyond a certain maximum value and can not begins the second collapse direct and grows in mass by accretion. Such a first core has a very long time-scale and seems to suffer from a non-axisymmetric evolution. In the usual case, we expect that the time-scale of a rotating first core becomes several thousand years and several percent of the prestellar cores contain a first core at least. Such a rotating first core has a small average luminosity of $L_{\rm core} = 0.003-0.03$($\dot{M}_{\rm core}/\dot{M}_{\rm ref}$) L$_{\odot}$.

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arXiv2005

Long-Term Evolution of Supernova Remnants in Magnetized Interstellar Medium

The evolution of supernova remnants (SNRs) is studied, with particular attention to the effect of magnetic fields with axisymmetric two-dimensional magnetohydrodynamical simulations. The evolution of magnetic SNRs is the same as non-magnetic ones in the adiabatic Sedov stage. After a thin shell is formed, the shell is driven by the pressure of the hot interior gas (bubble). Evolution in the pressure-driven snow-plow phase is much affected by the magnetic field. The shell sweeping the magnetic field lines thickens owing to the magnetic pressure force. After $5\times 10^5{\rm yr}$ - $2\times 10^6{\rm yr}$, the inner boundary of the thick shell begins to contract. This compresses the hot bubble radially and maintains its thermal pressure. Thus, the bubble forms a prolate spheroidal shape and becomes thinner and thinner, since it expands in a direction parallel to the magnetic field for $B_0 \gtrsim 3μ{\rm G}$. Finally, the bubble contracts. The porosity of the hot low-density gas in ISM is reduced, taking the effect of the magnetic field into account.

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arXiv2005

Collapse and Fragmentation of Rotating Magnetized Clouds. I. Magnetic Flux - Spin Relation

We discuss evolution of the magnetic flux density and angular velocity in a molecular cloud core, on the basis of three-dimensional numerical simulations, in which a rotating magnetized cloud fragments and collapses to form a very dense optically thick core of > 5 times 10 ^10 cm^-3 . As the density increases towards the formation of the optically thick core, the magnetic flux density and angular velocity converge towards a single relationship between the two quantities. If the core is magnetically dominated its magnetic flux density approaches 1.5 (n/5 times 10^10 cm^-3)^1/2 mG, while if the core is rotationally dominated the angular velocity approaches 2.57 times 10^-3, (n/5 times 10^10 cm^-3)^1/2 yr^-1, where n is the density of the gas. We also find that the ratio of the angular velocity to the magnetic flux density remains nearly constant until the density exceeds 5 times 10^10 cm^-3. Fragmentation of the very dense core and emergence of outflows from fragments are shown in the subsequent paper.

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arXiv2005

Collapse and Fragmentation of Rotating Magnetized Clouds. II. Binary Formation and Fragmentation of First Cores

Subsequent to Paper I, the evolution and fragmentation of a rotating magnetized cloud are studied with use of three-dimensional MHD nested-grid simulations. After the isothermal runaway collapse, an adiabatic gas forms a protostellar first core at the center of the cloud. When the isothermal gas is stable for fragmentation in a contracting disk, the adiabatic core often breaks into several fragments. Conditions for fragmentation and binary formation are studied. All the cores which show fragmentations are geometrically thin, as the diameter-to-thickness ratio is larger than 3. Two patterns of fragmentation are found. (1) When a thin disk is supported by centrifugal force, the disk fragments through a ring configuration (ring fragmentation). This is realized in a fast rotating adiabatic core as Omega >0.2 tau_ff^-1, where Omega and tau_ff represent the angular rotation speed and the free-fall time of the core, respectively. (2) On the other hand, the disk is deformed to an elongated bar in the isothermal stage for a strongly magnetized or rapidly rotating cloud. The bar breaks into 2 - 4 fragments (bar fragmentation). Even if a disk is thin, the disk dominated by the magnetic force or thermal pressure is stable and forms a single compact body. In either ring or bar fragmentation mode, the fragments contract and a pair of outflows are ejected from the vicinities of the compact cores. The orbital angular momentum is larger than the spin angular momentum in the ring fragmentation. On the other hand, fragments often quickly merge in the bar fragmentation, since the orbital angular momentum is smaller than the spin angular momentum in this case. Comparison with observations is also shown.

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arXiv2004

Low-Mass Star Formation, Triggered by Supernova in Primordial Clouds

The evolution of a gas shell, swept by the supernova remnant of a massive first generation star, is studied with H_2 and HD chemistry taken into account. When a first-generation star explodes as a supernova, H_2 and HD molecules are formed in the swept gas shell and effectively cool the gas shell to temperatures of 32 K - 154 K. If the supernova remnant can sweep to gather the ambient gas, the gas shell comes to be dominated by its self-gravity, and hence, is expected to fragment. Our result shows that for a reasonable range of temperatures (200 K - 1000 K) of interstellar gas, the formation of second-generation stars can be triggered by a single supernova or hypernova.

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arXiv2004

Molecular gas structure around an AGN with nuclear starburst: 3-D Non-LTE calculations of CO lines

We have performed three-dimensional, non-LTE radiative transfer calculations for 12CO and 13CO lines, applying them to our high-resolution hydrodynamic models of the `torus' around a SMBH in an active galactic nucleus. The hydrodynamic simulations reveal inhomogeneous and turbulent gas structure on a sub-pc scale in a circum-nuclear starburst region. Thick disks interlaced with filaments, clumps and holes are naturally formed due to the interplay among energy feedback from supernovae, self-gravity of the gas, galactic rotation, and radiative cooling. The intensity maps of the molecular lines for the circum-nuclear disks show a clumpy structure reflecting the intrinsic inhomogeneity and turbulent motion of the gas disk. The fine structure of the `torus' could be resolved in the nearby active galaxies using ALMA. We also found that the X-factor is not uniformly distributed in the central 100 pc region. The X-factor derived for 12CO(J=1-0) intensity depends strongly on the intensity, whereas the X for 12CO(3-2) is nearly constant over two orders of magnitude of the intensity. The suggested conversion factor for the molecular gas mass is X_CO (3-2) ~ 0.27 x 10^{20} cm^{-2} (K km s^{-1})^{-1}.

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arXiv2004

Directions of Outflows, Disks, Magnetic Fields, and Rotation of YSOs in Collapsing Molecular Cloud Cores

The collapse of slowly rotating molecular cloud cores threaded by magnetic fields is investigated by high-resolution numerical simulation. Outflow formation in the collapsing cloud cores is also followed. In the models examined, the cloud core and parent cloud rotate rigidly and are initially threaded by a uniform magnetic field. The simulations show that the cloud core collapses along the magnetic field lines. The magnetic field in the dense region of the cloud core rotates faster than that of the parent cloud as a consequence of spin-up of the central region during the collapse. The cloud core exhibits significant precession of the rotation axis, magnetic field, and disk orientation, with precession highest in the models with low initial field strength ($\lesssim 20 μ{\rm G}$). Precession in models with initial fields of $\sim 40 μ{\rm G}$ is suppressed by strong magnetic braking. Magnetic braking transfers angular momentum form the central region and acts more strongly on the component of angular momentum oriented perpendicular to the magnetic field. After the formation of an adiabatic core, outflow is ejected along the local magnetic field lines. Strong magnetic braking associated with the outflow causes the direction of angular momentum to converge with that of the local magnetic field, resulting in the convergence of the local magnetic field, angular momentum, outflow, and disk orientation by the outflow formation phase. The magnetic field of a young star is inclined at an angle of no more than $30^\circ$ from that of the parent cloud at initial field strengths of $\sim 20 μ{\rm G}$, while at an initial field strength of $\sim 40 μ{\rm G}$, the magnetic field of the young star is well aligned with that of the parent cloud.

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