Structure and stability of phase transition layers in the interstellar medium

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