ULTRAVIOLET SURVEY OF CO AND H2 IN DIFFUSE MOLECULAR CLOUDS: THE REFLECTION OF TWO PHOTOCHEMISTRY REGIMES IN ABUNDANCE RELATIONSHIPS

We carried out a comprehensive far-UV survey of (CO)-C-12 and H-2 column 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 H-2 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 cm(-3)) and higher density diffuse clouds. A plot of log N(CO) versus log N(H-2) shows that two power-law relationships are needed for a good fit of the entire sample, with a break located at log N(CO, cm(-2)) = 14.1 and log N(H-2) = 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 log N 14: 1, 13.0), CH+ versus H-2 (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 C-2 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.