CHEMICAL-TRANSITIONS FOR INTERSTELLAR C2 AND CN IN-CLOUD ENVELOPES

Observations were made of absorption from CH, C2, and CN toward moderately reddened stars in Sco OB2, Cep OB3, and Taurus/Auriga. For these directions, most of the reddening is associated with a single cloud complex, for example, the rho Ophiuchus molecular cloud, and as a result, the observations probe moderately dense material. When combined with available data for nearby directions, the survey provides the basis for a comprehensive analysis of the chemistry for these species. The chemical transitions affecting C2 and CN in cloud envelopes were analyzed. The depth into a cloud at which a transition takes place was characterized by tau(uv), the grain optical depth at 1000 angstrom. One transition at tau(uv) approximately 2, which arises from the conversion of C+ into CO, affects the chemistries for both molecules because of the key role this ion plays. A second one involving production terms in the CN chemistry occurs at tau(uv) of approximately 3; production via ion-molecule reactions with NH dominates at lower optical depths, while that via neutral-neutral reactions which C2 and CH is more important at larger values for tau(uv). The transition from photo-dissociation to chemical destruction takes place at tau(uv) approximately 4.5 for C2 and CN. The observational data for stars in Sco OB2, Cep OB3, and Taurus/Auriga were studied with chemical rate equations containing the most important production and destruction mechanisms. Because the sample of stars in Sco OB2 includes sight lines with A(v) ranging from 1-4 mag, sight lines dominated by photochemistry could be analyzed separately from those controlled by gas-phase destruction. The analysis yielded values for two poorly known rate constants for reactions involved in the production of CN; the reactions are C2 + N --> CN + C and C+ + NH --> all products. The other directions were analyzed with the inferred values. The predicted column densities for C2 and CN agree with the observed values to better than 50%, and in most instances 20%. When combining the estimates for density and temperature derived from chemical modeling and molecular excitation for a specific cloud, such as the rho Ophiuchus molecular cloud, the portion of the cloud envelope probed by C2 and CN absorption was found to be in pressure equilibrium.