The Role of Grains in Interstellar Chemistry

Abstract

Grains affect interstellar chemistry in a variety of ways. Most obviously, they extinguish starlight and thus protect molecules in cloud interiors from photodestruction. The grains themselves contain substantial proportions of particular elements which are therefore less readily available for gas phase reactions and for processing into molecules. Grains in dense clouds are known to accrete molecular mantles which may be further processed; the mantle material is ultimately returned to the gas, either near hot stars or when the clouds are dissipated. Molecular hydrogen, the key to all gas phase chemistry, is undoubtedly formed efficiently on grains, and a plausible mechanism can now be identified. Other molecules, too, form preferentially at surfaces. Finally, the destruction of grains via chemical erosion and by sputtering in shocks provides a substantial molecular contribution to the gas in local regions.