2007ApJ...667.1002S

We examine 20 diffuse and translucent Galactic sight lines and extract the column densities of the ¹²CO and ¹³CO isotopologues from their ultraviolet A-X absorption bands detected in archival Space Telescope Imaging Spectrograph data with λ/Δλ≥46,000. Five more targets with Goddard High-Resolution Spectrograph data are added to the sample that more than doubles the number of sight lines with published Hubble Space Telescope observations of ¹³CO. Most sight lines have ¹²CO-to-¹³CO isotopic ratios that are not significantly different from the local value of 70 for ¹²C/¹³C, which is based on millimeter-wave observations of rotational lines in emission from CO and H₂ CO inside dense molecular clouds, as well as on results from optical measurements of CH⁺. Five of the 25 sight lines are found to be fractionated toward lower ¹²C/¹³C values, while three sight lines in the sample are fractionated toward higher ratios, signaling the predominance of either isotopic charge exchange or selective photodissociation, respectively. There are no obvious trends of the ¹²CO-to-¹³CO ratio with physical conditions such as gas temperature or density, yet ¹²CO/¹³CO does vary in a complicated manner with the column density of either CO isotopologue, owing to varying levels of competition between isotopic charge exchange and selective photodissociation in the fractionation of CO. Finally, rotational temperatures of H₂ show that all sight lines with detected amounts of ¹³CO pass through gas that is on average colder by 20 K than the gas without ¹³CO. This colder gas is also sampled by CN and C₂molecules, the latter indicating gas kinetic temperatures of only 28 K, enough to facilitate an efficient charge exchange reaction that lowers the value of ¹²CO/¹³CO.