Formation of Turbulent and Magnetized Molecular Clouds via Accretion Flows of HI Clouds

Abstract

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 ~ 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.