Cooling behind mildly supersonic shocks in molecular clouds

Abstract

Collisions between stable, thermally supported gas clumps produce shock-compressed layers. However, these layers then undergo gravitational fragmentation only if the gas cools — on a dynamical time-scale or faster — to below its pre-shock temperature. Here we present an approximate analytic treatment of post-shock cooling which demonstrates that, under the conditions prevailing in molecular clouds, where the typical collision speeds are ~ 1 km s−1, cooling by dust is likely to be the dominant cooling mechanism, and is sufficiently fast to satisfy this requirement. Cooling by CO appears to be of secondary importance.