arXiv subjects

Kohji Tomisaka

Publications and source records attributed to Kohji Tomisaka.

81 records · Page 5Linked to original sources

arXiv2000

Outflow from YSO and Angular Momentum Transfer

Dynamical contraction of a slowly-rotating magnetized cloud is studied using the magnetohydrodynamical (MHD) simulations. In the isothermal stage ($n \la n_{\rm A} \sim 10^{10}{\rm cm}^{-3}$), the cloud evolves similarly to that expected from Larson-Penston self-similar solution and experiences the run-away collapse. However, after the central density exceeds $\sim n_{\rm A}$, an accretion disk is formed around the adiabatic core. Just outside the core, outflow is ejected by the effect of magnetic torque (magnetocentrifugal wind). Since $\sim$ 10% of the mass is ejected with almost all the angular momentum, the specific angular momentum of the protostellar core reduces to that observed in main-sequence stars.

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arXiv1999

Evolution of Angular Momentum Distribution during Star Formation

If the angular momentum of the molecular cloud core were conserved during the star formation process, a new-born star would rotate much faster than its fission speed. This constitutes the angular momentum problem of new-born stars. In this paper, the angular momentum transfer in the contraction of a rotating magnetized cloud is studied with axisymmetric MHD simulations. Owing to the large dynamic range covered by the nested-grid method, the structure of the cloud in the range from 10 AU to 0.1 pc is explored. First, the cloud experiences a run-away collapse, and a disk forms perpendicularly to the magnetic field, in which the central density increases greatly in a finite time-scale. In this phase, the specific angular momentum j of the disk decreases to $\simeq 1/3$ of the initial cloud. After the central density of the disk exceeds $\sim 10^{10}{\rm cm}^{-3}$, the infall on to the central object develops. In this accretion stage, the rotation motion and thus the toroidal magnetic field drive the outflow. The angular momentum of the central object is transferred efficiently by the outflow as well as the effect of the magnetic stress. In 7000 yr from the core formation, the specific angular momentum of the central $0.17M_\odot$ decreases a factor of 10^{-4} from the initial value (i.e. from $10^{20}{\rm cm^2 s^{-1}}$ to $10^{16}{\rm cm^2 s^{-1}}$).

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arXiv1998

Collapse-Driven Outflow in Star-Forming Molecular Cores

Dynamical collapses of magnetized molecular cloud cores are studied with magnetohydrodynamical simulations from the run-away collapse phase to the accretion phase. In the run-away collapse phase, a disk threaded by magnetic field lines is contracting due to its self-gravity and its evolution is well expressed by a self-similar solution. The central density increases greatly in a finite time scale and reaches a density at which an opaque core is formed at the center. After that, matter accretes to the newly formed core (accretion phase). In this stage, a rotationally supported disk is formed in a cloud core without magnetic fields. In contrast, the disk continues to contract in the magnetized cloud core, since the magnetic fields transfer angular momentum from the disk. Its rotation motion winds up the threading magnetic field lines. Eventually, strong toroidal magnetic fields are formed and begin to drive the outflow, even if there is no toroidal field component initially. Bipolar molecular outflows observed in protostar candidates are naturally explained by this model.

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arXiv1998

Superbubbles in magnetized interstellar media --- Blowout or confinement ? ---

The importance of the interstellar magnetic field is studied in relation to the evolutions of superbubbles with a three-dimensional (3D) numerical magnetohydrodynamical (MHD) simulation. A superbubble is a large supernova remnant driven by sequential supernova explosions in an OB association. Its evolution is affected by the density stratification in the galactic disk. After the size reaches 2--3 times the density scale-height, the superbubble expands preferentially in the $z$-direction. Finally it can punch out the gas disk (blow-out). On the other hand, the magnetic field running parallel to the galactic disk has an effect to prevent from expanding in the direction perpendicular to the field. The density stratification and the magnetic fields have completely opposite effects on the evolution of the superbubble. We present results of 3D MHD simulation in which both effects are included. As a result, it is concluded that when the magnetic field has a much larger scale-height than the density, even for a model that the bubble would blow out from the disk if the magnetic field were absent, the magnetic field with the strength of 5 $μ$G can confine the bubble in $|z| \alt 300$ pc for $\simeq$20 Myr (confinement). In a model that the field strength decreases in the halo in proportion to $B \propto ρ^{1/2}$, the superbubble eventually blows out like a model of B=0 even if the magnetic field in the mid-plane is as strong as $B=5μ$G.

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arXiv1997

Recent X-Ray Observations of SN1986J with ASCA and ROSAT

We present ASCA and ROSAT observations of SN 1986J covering the period 1991 August to 1996 January. From observations with the ROSAT HRI and PSPC, we find that the 0.5-2.5 keV flux decreased proportional to $t^{-2}$ during this period; the ASCA data are consistent with this result and extend it to the 2-10 keV band. ASCA spectra from 1994 January and 1996 January are consistent with thermal emission from a solar metallicity plasma at an equilibrium temperature kT = 5-7.5 keV, somewhat hotter than that observed from other X-ray supernovae. These spectra also show a clear Fe K emission line at 6.7 keV with FWHM < 20,000 km/s (90% confidence). This limit on the line width is consistent with the reverse shock model of Chevalier & Fransson (1994), but does not rule out the clumpy wind model of Chugai (1993).

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arXiv1996

Accretion in Gravitationally Contracting Clouds

Accretion flow in a contracting magnetized isothermal cloud was studied using magnetohydrodynamical simulations and a nested grid technique. First, the interstellar magnetized cloud experiences a ``runaway collapse'' phase, in which the central density increases drastically within a finite time scale. Finally, it enters an accretion phase, in which inflowing matter accretes onto a central high-density disk or a new-born star. We found that the accretion rate reaches (4 -- 40) $\times c_s^3/G$, where $c_s$ and $G$ represent the isothermal sound speed and the gravitational constant, respectively. This is much larger than the standard accretion rate of $0.975c_s^3/G$ for a hydrostatic isothermal spherical cloud (Shu 1977, AAA19.065.044). Due to the effect of an extra infall velocity achieved in the runaway phase ($\sim 2 c_s$), the accretion rate is boosted. This rate declines with time in contrast to Shu's solution, but keeps $\gtsim 2.5 c_s^3/G$. The observed gas infall rate around proto-stars such as L1551 IRS 5 and HL Tau is also discussed.

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arXiv1994

Collapse and Fragmentation of Cylindrical Magnetized Clouds. II. Simulation with Nested Grid Scheme

Fragmentation process in a cylindrical magnetized cloud is studied with the nested grid method. The nested grid scheme use 15 levels of grids with different spatial resolution overlaid subsequently, which enables us to trace the evolution from the molecular cloud density $\sim 100 {\rm cm}^{-3}$ to that of the protostellar disk $\sim 10^{10} {\rm cm} ^{-3}$ or more. Fluctuation with small amplitude grows by the gravita- tional instability. It forms a disk perpendicular to the magnetic fields which runs in the direction parallel to the major axis of the cloud. Matter accrets on to the disk mainly flowing along the magnetic fields and this makes the column density increase. The radial inflow, whose velocity is slower than that perpendicular to the disk, is driven by the increase of the gravity. While the equation of state is isothermal and magnetic fields are perfectly coupled with the matter, which is realized in the density range of $ρ\lesssim 10^{10} {\rm cm}^{-3}$, never stops the contraction. The structure of the contracting disk reaches that of a singular solution as the density and the column density obey $ρ(r)\propto r^{-2}$ and $σ(r) \propto r^{-1}$, respectively. The magnetic field strength on the mid-plane is proportional to $ρ(r)^{1/2}$ and further that of the center ($B_c$) evolves as proportional to the square root of the gas density ($\propto ρ_c^{1/2}$). It is shown that isothermal clouds experience ``run-away'' collapses. The evolution after the equation of state becomes hard is also discussed.

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arXiv1993

Collapse and Fragmentation of Magnetized Cylindrical Clouds

Gravitational collapse of the cylindrical elongated cloud is studied by numerical magnetohydrodynamical simulations. In the infinitely long cloud in hydrostatic configuration, small perturbations grow by the gravitational instability. The most unstable mode indicated by a linear perturbation theory grows selectively even from a white noise. The growth rate agrees with that calculated by the linear theory. First, the density-enhanced region has an elongated shape, i.e., prolate spheroidal shape. As the collapse proceeds, the high-density fragment begins to contract mainly along the symmetry axis. Finally, a spherical core is formed in the non-magnetized cloud. In contrast, an oblate spheroidal dense disk is formed in a cloud in which the magnetic pressure is nearly equal to the thermal one. The radial size of the disk becomes proportional to the initial characteristic density scale-height in the r-direction. As the collapse proceeds, a slowly contracting dense part is formed (approximately < 10% in mass) inside of the fast contracting disk. And this is separated from other part of the disk whose inflow velocity is accelerated as reaching the center of the core. From arguments on the Jeans mass and the magnetic critical mass, it is concluded that the fragments formed in a cylindrical elongated cloud can not be supported against the self- gravity and it will eventually collapse.

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arXiv1992

Extended Hot Gas Halos Around Starburst Galaxies

Reanalysis of Einstein IPC data and new observations from the GINGA LAC indicate the presence of extended X-ray emission (10-50 kpc) around the starburst galaxy M82. Here we model this emission by calculating numerical hydrodynamic simulations of the starburst event to much later times and larger scales than previously considered. For our models, we adopt a supernova rate of 0.1 ${\rm yr}^{-1}$, and an extended low-density static halo that is bound to the galaxy. There are three stages to the evolution of the wind-blown bubble and the propagation of the shock front: the bubble expands in an almost uniform density disk gas, with a deceleration of the shock front ($t \alt $ 3.6 Myr); breakout from the disk and the upward acceleration of the shock front (3.6 Myr $\alt t \alt$ 18 Myr); propagation into the halo, leading to a more spherical system and shock deceleration (18 Myr $\alt t$). For a halo density of $10^{-3} {\rm cm}^{-3}$, the outflow reaches a distance of 40-50 kpc from the center of the starburst galaxy in 50 Myr. We calculate the time evolution of the X-ray luminosity and find that the extended starburst emits $3\times 10^{39}\lcgs$ to $10^{40}\lcgs$ in the GINGA LAC band and $\sim 10^{41}\lcgs$ in the Einstein or ROSAT HRI band. The degree of the ionization equilibrium in the outflow and its effect on the iron K$α$ line emission are discussed.

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