2008ApJ...687.1075S

We carried out a comprehensive far-UV survey of ¹²CO and H₂column densities along diffuse molecular Galactic sight lines. This sample includes new measurements of CO from HST spectra along 62 sight lines and new measurements of H₂ from FUSE data along 58 sight lines. In addition, high-resolution optical data were obtained at the McDonald and European Southern Observatories, yielding new abundances for CH, CH⁺, and CN along 42 sight lines to aid in interpreting the CO results. These new sight lines were selected according to detectable amounts of CO in their spectra and provide information on both lower density (≤100/cm3) and higher density diffuse clouds. A plot of logN(CO) versus logN(H₂) shows that two power-law relationships are needed for a good fit of the entire sample, with a break located at logN(CO,cm^–2)=14.1 and logN(H₂)=20.4, corresponding to a change in production route for CO in higher density gas. Similar logarithmic plots among all five diatomic molecules reveal additional examples of dual slopes in the cases of CO versus CH (break at logN=14.1, 13.0), CH⁺ versus H₂(13.1, 20.3), and CH⁺ versus CO (13.2, 14.1). We employ both analytical and numerical chemical schemes in order to derive details of the molecular environments. In the denser gas, where C₂ and CN molecules also reside, reactions involving C⁺ and OH are the dominant factor leading to CO formation via equilibrium chemistry. In the low-density gas, where equilibrium chemistry studies have failed to reproduce the abundance of CH⁺, our numerical analysis shows that nonequilibrium chemistry must be employed for correctly predicting the abundances of both CH⁺ and CO.