Toward understanding the formation of molecular clouds

The analysis of the thermal and chemical equilibrium states of the interstellar medium shows the existence of two stable branches in the equilibrium states, that correspond to warm gases and cold gases. The dynamical evolution through thermal instability in the post-shock layer becomes important for the transition from warm gases to cold gases, because the shock waves are frequently generated by supernovae in the Galaxy. We have investigated the propagation of a shock wave into an interstellar medium by two-dimensional numerical hydrodynamic calculations with cooling and thermal conduction. We present results from the first high resolution two-dimensional calculations ever to follow the fragmentation due to thermal instability in shock-compressed layer. We use realistic thermal conduction coefficient to resolve the `Field length' that is the critical wavelength for the thermal instability. We find that geometrically thin cooling layer behind the shock front fragments into small cloudlets. The cloudlets have supersonic velocity dispersion in the warm neutral medium in which the fragments are embedded as cold condensations.