5 - Shock chemistry in diffuse clouds

Summary

Introduction

The high observed column densities of CH⁺, one of the first identified (Douglas and Herzberg 1941) interstellar molecules, and of CO apparently indicate that existing static, equilibrium models do not provide adequate descriptions of the natures of diffuse molecular interstellar clouds. (See Chapter 3.) It has been argued that velocity structures in lines formed in such clouds provide evidence for the existence of shocks in them (e.g. Crutcher (1979), but see the detailed assessment by Langer in Chapter 4). If such shocks do exist, they will drive the production of detectable column densities of a number of chemical species.

The chemistry in shocked gas can be exceptionally rich since many reactions which, because they are endothermic or have activation barriers, are unimportant in cool, static gas, can proceed in shocked gas. For instance, the endothermic reactions C⁺ + H₂ → CH⁺ + H (Elitzur and Watson 1978a) and S⁺ + H₂ → SH⁺ + H (Millar et al. 1986) can initiate hydrogen abstraction sequences in shocked gas but are unimportant in static, cool diffuse clouds. A neutral–neutral sequence (Aannestad 1973) which is of no relevance to low temperature chemistry but which plays a major role in shock chemistry is O + H₂ → OH + H; OH + H₂ → H₂O + H. The fractional abundances of CH⁺ and OH are high in some diffuse cloud shocks, and SH⁺ may serve as a diagnostic of shocks.

Collisionally induced rotational excitation of molecular hydrogen can also occur in diffuse cloud shocks.