The Role of Thermal Instability in Interstellar Medium

Abstract

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 from warm gases to cold molecular gases, because the shock waves are frequently generated by supernovae in the Galaxy. The mechanism for maintaining the turbulent motion in two‐phase medium is analyzed further by identifying the dynamical instability of the transition layer between WNM and CNM, that has analogy to Darrieus‐Landau Instability of flame fronts and the corrugation instability of MHD slow shocks. Once the total column density of the ensemble of cold clouds becomes larger than the critical value (∼ 1021cm−2), the two‐phase medium is expected to become one phase medium with the cooling timescale. This process is not well understood and remains to be studied. Attempts to compare the numerical results of dynamical calculations with observation are suggested.