COOLING BEHIND MILDLY SUPERSONIC SHOCKS IN MOLECULAR CLOUDS

Collisions between stable, thermally supported gas clumps produce shoc k-compressed layers. However, these layers then undergo gravitational fragmentation only if the gas cools - on a dynamical time-scale or fas ter - to below its pre-shock temperature. Here we present an approxima te analytic treatment of post-shock cooling which demonstrates that, u nder the conditions prevailing in molecular clouds, where the typical collision speeds are similar to 1 km s(-1), cooling by dust is likely to be the dominant cooling mechanism, and is sufficiently fast to sati sfy this requirement. Cooling by CO appears to be of secondary importa nce.