Chemistry in interstellar flows

Dissipation in flows often heats molecular gas to temperatures at which many reactions that are endothermic or that have activation barriers can proceed. H2 observations have been used to diagnose the conditions in flows in molecular clouds, but theoretical studies of the H2 level population problem are very incomplete. Much of the H2 emission and many of the chemical species formed arise in the precursor regions of MHD shocks; several quite general differences between the chemistries in precursor zones and in high T regions behind ordinary hydrodynamic shocks exist. The abundance of CH+ in MHD shocks in diffuse molecular clouds should exceed that in ordinary hydrodynamic shocks but shock formation models for the observed CH+ abundances face several difficulties. (author)