The Role of Thermal Instability in Interstellar Medium

Our understanding on the physical processes in the transition between warm neutral medium (WNM) and cold neutral medium (CNM) is dramatically increased in the last few years. This article reviews the role of thermal instability in interstellar medium. First we explain the basic property of thermal instability in terms of linear stability analysis. Then we analyze the propagation of a shock wave into WNM or CNM by taking into account radiative heating/cooling, thermal conduction, and physical viscosity, in one‐, two‐, and three‐dimensional magnetohydrodynamical simulations. The results show that the thermal instability in the post‐shock gas 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 sub‐sonic 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 ...