Hubble Space Telescope Survey of Interstellar super(12)CO/ super(13)CO in the Solar Neighborhood

• Published in: The Astrophysical journal Vol. 667; no. 2; pp. 1002 - 1016
• Main Authors: Sheffer, Y, Rogers, M, Federman, SR, Lambert, D L, Gredel, R
• Format: Journal Article
• Language: English
• Published: 01-10-2007
• ISSN: 0004-637X

We examine 20 diffuse and translucent Galactic sight lines and extract the column densities of the super(12)CO and super(13)CO isotopologues from their ultraviolet A-X absorption bands detected in archival Space Telescope Imaging Spectrograph data with lambda / Delta lambda greater than or equal to 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 super(13)CO. Most sight lines have super(12)CO-to- super(13)CO isotopic ratios that are not significantly different from the local value of 70 for super(12)C/ super(13)C, which is based on millimeter-wave observations of rotational lines in emission from CO and H sub(2)CO inside dense molecular clouds, as well as on results from optical measurements of CH super(+). Five of the 25 sight lines are found to be fractionated toward lower super(12)C/ super(13)C 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 super(12)CO-to- super(13)CO ratio with physical conditions such as gas temperature or density, yet super(12)CO/ super(13)CO 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 H sub(2) show that all sight lines with detected amounts of super(13)CO pass through gas that is on average colder by 20 K than the gas without super(13)CO. This colder gas is also sampled by CN and C sub(2) molecules, the latter indicating gas kinetic temperatures of only 28 K, enough to facilitate an efficient charge exchange reaction that lowers the value of super(12)CO/ super(13)CO.