Molecular formation on interstellar grains

The temperature dependence of the relative abundance of complex molecules formed on interstellar grains is examined in this paper. H, C, N, and O atoms of cosmic abundance ratio are assumed to stick onto the grain surface from the gas phase and be absorbed semi-chemically. Absorbed atoms migrate on the surface of grain passing over the potential barriers, and grow to more complex radicals or molecules in each encounter with other radicals or atoms. When the dust temperature Tsub(d) is below 20 K, simple and fully saturated molecules such as H2, H2O, NH3, and CH4 are formed and only negligible amounts of other molecules are formed. Above 50 K, complex and unsaturated molecules are formed. The observed variety of molecules is shown to be consistent with the abundance predicted for a grain temperature between 40 and 50K. (auth.)