Hubble Space Telescope survey of interstellar 12CO/13CO in the solar neighborhood.
SHEFFER Y., ROGERS M., FEDERMAN S.R., LAMBERT D.L. and GREDEL R.
Abstract (from CDS):
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.
ISM: Abundances - ISM: Molecules - Ultraviolet: ISM