FORMATION OF TURBULENT AND MAGNETIZED MOLECULAR CLOUDS VIA ACCRETION FLOWS OF H I CLOUDS

Abstract

Using 3D MHD simulation with the effects of radiative cooling/heating,\nchemical reactions, and thermal conduction, we investigate the formation of\nmolecular cloud in the ISM. We consider the formation of molecular cloud by\naccretion of the HI clouds as suggested in recent observations. The simulation\nshows that the initial HI medium is compressed and piled up behind the shock\nwaves induced by the accretion flows. Since the initial medium is highly\ninhomogeneous as a consequence of the thermal instability, the formed molecular\ncloud becomes very turbulent owing to the development of the Richtmyer-Meshkov\ninstability. The structure of the post shock region is composed of dense cold\ngas (T<100 K) and diffuse warm gas (T>1,000 K), which are spatially well mixed\nowing to the turbulence. Because the energy source of the turbulence is the\naccretion flows, the turbulence is highly anisotropic biased toward the\ndirection of accretion flows. The kinetic energy of the turbulence dominates\nthe thermal, magnetic, and gravitational energies in the total 10 Myr\nevolution. However, the kinetic energy measured by using the\nCO-fraction-weighted density is comparable to the other energies. This suggests\nthat the true kinetic energy of turbulence in molecular cloud as a hole can be\nmuch larger than the kinetic energy of turbulence estimated by line-width of\nmolecular emissions. The clumps in the molecular cloud show statistically\nhomogeneous evolution as follows: The typical plasma beta of the clumps is\nroughly constant <beta>~ 0.4. The size-velocity dispersion relation show dv ~\n1.5 km s^{-1} (l/1 pc)^{0.5}, irrespective of the density. The clumps evolve\ntoward magnetically supercritical, gravitationally unstable cores. The clumps\nseem to evolve into cores that satisfy the condition for fragmentation into\nbinary. These statistical properties may provide the initial condition of star\nformation.\n