On the origin of NH in diffuse interstellar clouds

Abstract

The observation by Meyer & Roch of NH in the interstellar clouds towards ζ Per and HD 27778 cannot be explained with conventional gas-phase chemistry models. A simple non-equilibrium model for the ζ Per cloud, which incorporates the grainsurface production of NH and OH or, alternatively, NH3 and H2O, is able to reproduce the abundances of all observed species (except CH+) quite accurately. Moreover, chemical models which include grain-surface reactions can reproduce the observed abundance not only of NH but also of CN, which is efficiently formed at low temperatures, initiated by the reaction of NH with C+. Pure gas-phase models and cloud interface models, in which NH and CH+ are formed in a warm and tenuous environment, fail to explain the observed high abundance of CN. Hence the observation of NH in ζ Per and HD 27778 provides evidence for the presence of grainsurface reactions leading to molecules other than H2. It is predicted that NH2 and NH3 should have abundances not much below that of NH if NH3 instead of NH is formed on grains. With or without surface reactions, the column densities of H20 and C2H are expected to be about 1013 cm–2, and these molecules may be detectable in the ζ Per cloud.