Magnetohydrodynamic shocks in diffuse clouds. I - Chemical processes

The chemistry of carbon- and oxygen-bearing species in MHD shock waves in diffuse interstellar clouds is examined, and the heating and cooling mechanisms in the shocked gas are discussed. Rate coefficients for endothermic and exothermic ion-neutral reactions are computed for conditions where the mean velocities of the ions and neutrals differ, resulting in a non-Maxwellian distribution of relative velocities, and energy-dependent cross sections are adopted for the principal reactions. Confirmation of the important effect on the ion abundance resulting from the endothermic reaction of C(+) with H2, followed by fast dissociative recombination, is obtained. Results for an illustrative model shock demonstrate that significant quantities of CH, OH, CH(+), CH2(+), CH3(+), and HCO(+) can be produced in MHD shocks in diffuse clouds, and that MHD shocks may play an important role in determining the overall abundance of CH(+).