Instability of Evaporation Fronts in the Interstellar Medium
  The neutral component of the interstellar medium is segregated into the cold
neutral medium (CNM) and warm neutral medium (WNM) as a result of thermal
instability. It was found that a plane-parallel CNM-WNM evaporation interface,
across which the CNM undergoes thermal expansion, is linearly unstable to
corrugational disturbances, in complete analogy with the Darrieus-Landau
instability (DLI) of terrestrial flames. We perform a full linear stability
analysis as well as nonlinear hydrodynamic simulations of the DLI of such
evaporation fronts in the presence of thermal conduction. We find that the DLI
is suppressed at short length scales by conduction. The length and time scales
of the fastest growing mode are inversely proportional to the evaporation flow
speed of the CNM and its square, respectively. In the nonlinear stage, the DLI
saturates to a steady state where the front deforms to a finger-like shape
protruding toward the WNM, without generating turbulence. The evaporation rate
at nonlinear saturation is larger than the initial plane-parallel value by a
factor of 2.4 when the equilibrium thermal pressure is 1800 k_B cm^-3 K. The
degrees of front deformation and evaporation-rate enhancement at nonlinear
saturation are determined primarily by the density ratio between the CNM and
WNM. We demonstrate that the Field length in the thermally unstable medium
should be resolved by at least four grid points to obtain reliable numerical
outcomes involving thermal instability.
