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We examine 20 diffuse and translucent Galactic sight lines and extract thecolumn densities of the ^12CO and ^13CO isotopologues from their ultravioletA--X absorption bands detected in archival Space Telescope Imaging Spectrographdata with lambda/Deltalambda geq 46,000. Five more targets with GoddardHigh-Resolution Spectrograph data are added to the sample that more thandoubles the number of sight lines with published Hubble Space Telescopeobservations of ^13CO. Most sight lines have 12-to-13 isotopic ratios that arenot significantly different from the local value of 70 for ^12C/^13C, which isbased on mm-wave observations of rotational lines in emission from CO and H_2COinside dense molecular clouds, as well as on results from optical measurementsof CH^+. Five of the 25 sight lines are found to be fractionated toward lower12-to-13 values, while three sight lines in the sample are fractionated towardhigher ratios, signaling the predominance of either isotopic charge exchange orselective photodissociation, respectively. There are no obvious trends of the^12CO-to-^13CO ratio with physical conditions such as gas temperature ordensity, yet ^12CO/^13CO does vary in a complicated manner with the columndensity of either CO isotopologue, owing to varying levels of competitionbetween isotopic charge exchange and selective photodissociation in thefractionation of CO. Finally, rotational temperatures of H_2 show that allsight lines with detected amounts of ^13CO pass through gas that is on averagecolder by 20 K than the gas without ^13CO. This colder gas is also sampled byCN and C_2 molecules, the latter indicating gas kinetic temperatures of only 28K, enough to facilitate an efficient charge exchange reaction that lowers thevalue of ^12CO/^13CO.

Hubble Space Telescope Survey of Interstellar ^12CO/^13CO in the Solar Neighborhood