The formation of carbon monoxide and the thermal balance in interstellar clouds.

A chemical scheme is presented for the formation of carbon monoxide in interstellar clouds of low density which suggests that the conversion of C into CO is substantial at depths corresponding to visual extinctions exceeding unity. At temperatures near 80 K, the main source of CO is the reaction of ionized carbon with the OH radical, but at low temperatures a sequence initiated by the radiative association of ionized carbon and molecular hydrogen is more efficient. Because of the enhanced cooling at low temperatures, the conversion of C to CO affects significantly the evolution of the cloud. We have solved the coupled time-dependent chemical and energy equations for the isochoric cooling of a cloud. The production of CO leads to the formation of a cold region in the cloud that may be unstable, and we suggest that the contraction of an interstellar cloud depends critically on the formation of CO and of other interstellar molecules. Subject headings: atomic and molecular processes - interstellar matter - molecules, interstellar