Shocks in Diffuse and Dense Molecular Clouds

Abstract

The chemistry in postshock gas in interstellar clouds is particularly rich because reactions which are endothermic or which have activation barriers proceed more readily in gas heated to T ≳ 10³ K than in cooler gas; in particular, the formation of sulfur-bearing and silicon-bearing species is efficient in postshock regions. Steady multifluid hydromagnetic models have been constructed for shocks in diffuse and dense molecular clouds. Considerable effort has gone into the study of the abundance of rotationally excited H₂, of CH⁺, and of OH, which is an important diagnostic of the low energy cosmic ray distribution, in diffuse cloud shocks. Grain-neutral drag is important in determining the structures of shocks in dense clouds. It has been incorporated in models of the Orion shock, but more sophisticated treatments of it may be required for situations in which there are many small grains. The observation at different positions of a nearly constant ratio of the strengths of the H₂ v = 0-0 S(13) and v = 1-0 0(7) H₂ lines may be difficult to understand with models of the type constructed for the Orion shock. The gas behind some dissociative shocks may be thermally unstable. High density ultraviolet heated static dissociation zones have spectra in which the H₂ infrared line ratios are similar to those appropriate for shocked gas. In estimating the energy content of high velocity molecular flows, it is important to integrate properly over the observed line profiles; when one does so, one concludes that the flows may be momentum driven by radiation driven stellar winds.