The Formation of Molecular Clouds

Abstract

Abstract The propagation of a shock wave into atomic interstellar medium is analyzed by taking into account radiative heating/cooling, thermal conduction, and physical viscosity, in three-dimensional magnetohydrodynamical simulations. The results show that the thermal instability in the post-shock gas in the interstellar medium produces high-density molecular cloudlets embedded in warm neutral medium. The molecular cloudlets have velocity dispersion which is supersonic with respect to the sound speed of the cold medium but is subsonic with respect to the warm medium. The dynamical evolution driven by thermal instability in the post-shock layer is an important basic process for the transition from warm gases to cold molecular gases, because the shock waves are frequently generated by supernovae in the Galaxy.