Formation of Turbulent and Magnetized Molecular Clouds via Accretion
  Flows of HI Clouds
  Using 3D MHD simulation with the effects of radiative cooling/heating,
chemical reactions, and thermal conduction, we investigate the formation of
molecular cloud in the ISM. We consider the formation of molecular cloud by
accretion of the HI clouds as suggested in recent observations. The simulation
shows that the initial HI medium is compressed and piled up behind the shock
waves induced by the accretion flows. Since the initial medium is highly
inhomogeneous as a consequence of the thermal instability, the formed molecular
cloud becomes very turbulent owing to the development of the Richtmyer-Meshkov
instability. The structure of the post shock region is composed of dense cold
gas (T<100 K) and diffuse warm gas (T>1,000 K), which are spatially well mixed
owing to the turbulence. Because the energy source of the turbulence is the
accretion flows, the turbulence is highly anisotropic biased toward the
direction of accretion flows. The kinetic energy of the turbulence dominates
the thermal, magnetic, and gravitational energies in the total 10 Myr
evolution. However, the kinetic energy measured by using the
CO-fraction-weighted density is comparable to the other energies. This suggests
that the true kinetic energy of turbulence in molecular cloud as a hole can be
much larger than the kinetic energy of turbulence estimated by line-width of
molecular emissions. The clumps in the molecular cloud show statistically
homogeneous evolution as follows: The typical plasma beta of the clumps is
roughly constant <beta>~ 0.4. The size-velocity dispersion relation show dv ~
1.5 km s^{-1} (l/1 pc)^{0.5}, irrespective of the density. The clumps evolve
toward magnetically supercritical, gravitationally unstable cores. The clumps
seem to evolve into cores that satisfy the condition for fragmentation into
binary. These statistical properties may provide the initial condition of star
formation.
