Kazunari Iwasaki

BIMODALITY OF CIRCUMSTELLAR DISK EVOLUTION INDUCED BY THE HALL CURRENT

The formation process of circumstellar disks is still controversial because of the interplay of complex physical processes that occurs during the gravitational collapse of prestellar cores. In this study, we investigate the effect of the Hall current term on the formation of the circumstellar disk using three-dimensional simulations. In our simulations, all non-ideal effects, as well as the radiation transfer, are considered. The size of the disk is significantly affected by a simple difference in the inherent properties of the prestellar core, namely whether the rotation vector and the magnetic field are parallel or anti-parallel. In the former case, only a very small disk () is formed. On the other hand, in the latter case, a massive and large () disk is formed in the early phase of protostar formation. Since the parallel and anti-parallel properties do not readily change, we expect that the parallel and anti-parallel properties are also important in the subsequent disk evolution and the difference between the two cases is maintained or enhanced. This result suggests that the disk size distribution of the Class 0 young stellar objects is bimodal. Thus, the disk evolution can be categorized into two cases and we may call the parallel and anti-parallel systems Ortho-disk and Para-disk, respectively. We also show that the anti-rotating envelopes against the disk rotation appear with a size of . We predict that the anti-rotating envelope will be found in the future observations.

The formation and destruction of molecular clouds and galactic star formation - An origin for the cloud mass function and star formation efficiency

We describe an overall picture of galactic-scale star formation. Recent high-resolution magneto-hydrodynamical simulations of two-fluid dynamics with cooling/heating and thermal conduction have shown that the formation of molecular clouds requires multiple episodes of supersonic compression. This finding enables us to create a scenario in which molecular clouds form in interacting shells or bubbles on a galactic scale. First we estimate the ensemble-averaged growth rate of molecular clouds over a timescale larger than a million years. Next we perform radiation hydrodynamics simulations to evaluate the destruction rate of magnetized molecular clouds by the stellar FUV radiation. We also investigate the resultant star formation efficiency within a cloud which amounts to a low value (a few percent) if we adopt the power-law exponent -2.5 for the mass distribution of stars in the cloud. We finally describe the time evolution of the mass function of molecular clouds over a long timescale (>1Myr) and discuss the steady state exponent of the power-law slope in various environments.

Effects of Ohmic and ambipolar diffusion on formation and evolution of first cores, protostars, and circumstellar discs

We investigate the formation and evolution of a first core, protostar, and circumstellar disc with a three-dimensional non-ideal (including both Ohmic and ambipolar diffusion) radiation magnetohydrodynamics simulation. We found that the magnetic flux is largely removed by magnetic diffusion in the first core phase and that the plasma

GRAVITATIONAL FRAGMENTATION OF EXPANDING SHELLS. I. LINEAR ANALYSIS

\beta

Smoothed particle magnetohydrodynamics with a Riemann solver and the method of characteristics

of the centre of the first core becomes large,

Alfvén wave amplification and self-containment of cosmic rays escaping from a supernova remnant

\beta>10^4

An explicit scheme for ohmic dissipation with smoothed particle magnetohydrodynamics

. Thus, proper treatment of first core phase is crucial in investigating the formation of protostar and disc. On the other hand, in an ideal simulation,

GRAVITATIONAL FRAGMENTATION OF EXPANDING SHELLS. II. THREE-DIMENSIONAL SIMULATIONS

\beta\sim 10

Self-sustained Turbulence without Dynamical Forcing: A Two-dimensional Study of a Bistable Interstellar Medium

at the centre of the first core. The simulations with magnetic diffusion show that the circumstellar disc forms at almost the same time of protostar formation even with a relatively strong initial magnetic field (the value for the initial mass-to-flux ratio of the cloud core relative to the critical value is

Fragmentation of a dynamically condensing radiative layer

\mu=4