Shielding of CO from dissociating radiation in interstellar clouds

We investigate the photodissociation of CO in interstellar clouds in the light of recent laboratory studies which suggest that line rather than continuum processes dominate its dissociation by ultraviolet radiation. Using a simple radiative transfer model, we estimate the shielding of representative dissociating bands including self-shielding, mutual shielding between different isotopes, and near coincidences with strong lines of H2. Each of these processes materially affects the photodestruction rates of the various isotopic species in the transition regions of molecular clouds. We combine these results with an appropriate gas phase chemical model to determine how the abundances of the CO isotopes vary with depth into the cloud. We find that self-shielding and mutual shielding cause significant variations in isotopic ratios. In addition fractionation enhances species containing 13C. The relationship between the column densities of CO and H2 is found to vary for the different isotopes and to be sensitive to local conditions