Structure and Stability of Phase Transition Layers in Interstellar Medium

We analyze the structure and stability of the transition layer (or front) that connect the cold neutral medium and warm neutral medium in the plane-parallel geometry. Such fronts appear in recent numerical simulations of thermally bistable interstellar medium. The front becomes the evaporation or condensation front depending on surrounding pressure. The stability analysis was done by long and short wavelength approximations. We find that the plane-parallel evaporation front is unstable under the corrugational deformation, whereas the condensation front seems to be stable. The instability is analogous to the Darrieus-Landau instability of the combustion front. The growth rate of the instability is proportional to the evaporation flow speed and corrugation wavenumber for the modes with wavelength much longer than the thickness of the front, and it is suppressed at the scale approximately equal to the thickness of the front. The timescale of the instability is smaller than the cooling timescale of the warm neutral medium (∼ 1 Myr), and can be as small as the cooling timescale of the cold neutral medium (∼ 0.01 − 0.1 Myr). Thus, the instability should be one of the processes for driving the interstellar turbulence.