The Equilibrium of CH and CH+ in Interstellar Space

Abstract

The theory of surface reactions of McCrea and McNally is extended to the formation of CH at the surfaces of charged interstellar grains. The rate coefficient for this reaction is calculated to be 1.6 × 10 ⁻¹⁶ cm ³ sec ⁻¹ at 100 °K. The equilibrium of CH and CH ⁺ is then investigated for three different radiation fields. It is shown that surface reactions can account for the observed amounts of CH and CH ⁺ provided the intensity of the interstellar radiation in the neighbourhood of the Lyman limit is sufficiently weak. In particular it is shown that the radiation field recently found by Lambrecht and Zimmermann is too intense in this region to allow the reaction. Chemical exchange reactions are eliminated as possible molecule-forming reactions and, since Bates and Spitzer have shown that radiative association is too slow to account for the observed densities, only surface reactions have a chance of being sufficiently fast to produce molecules. It is further shown that the interstellar grains are probably less dense than 1 g cm ⁻³ if there are to be many free atoms of carbon in interstellar space.