Surface hydrogenation in diffuse and translucent interstellar clouds

Abstract

We investigate the eects of grain-surface hydrogenation reactions on gas phase chemical models of diuse and translucent clouds in the interstellar medium. Models in which gas phase species freeze out on to dust without release do not match observed column densities well. Expanding on previous work we extend our models to include the release of C, N, O, S and CO following hydrogenation on the grain surface. The results show that such mechanisms do improve the ability of chemical models to reproduce observed abundances, not only through the release of otherwise frozen-out species but also because of the additional hydrogenating reactions. For example, the predicted column densities of NH in diuse clouds and H2 Sa nd NH 3 in translucent clouds match observations better in models with grain-surface hydrogenation.