We carried out a comprehensive far-UV survey of 12 CO 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 À3) 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 À2 ) ¼ 14:1 and log N (H 2 ) ¼ 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 ¼ 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.

We report on strong H 2 and CO absorption from gas within the host galaxy of gamma-ray burst (GRB) 080607. Analysis of our Keck/LRIS afterglow spectrum reveals a very large H I column density (N H I = 10 22.70±0.15 cm −2 ) and strong metal-line absorption at z GRB = 3.0363 with a roughly solar metallicity. We detect a series of A − X bandheads from CO and estimate N (CO) = 10 16.5±0.3 cm −2 and T CO ex > 100 K. We argue that the high excitation temperature results from UV pumping of the CO gas by the GRB afterglow. Similarly, we observe H 2 absorption via the Lyman-Werner bands and estimate N H2 = 10 21.2±0.2 cm −2 with T H2 ex = 10-300 K. The afterglow photometry suggests an extinction law with R V ≈ 4 and A V ≈ 3.2 mag and requires the presence of a modest 2175Å bump. Additionally, modeling of the Swift/XRT X-ray spectrum confirms a large column density with N H = 10 22.58±0.04 cm −2 . Remarkably, this molecular gas has extinction properties, metallicity, and a CO/H 2 ratio comparable to those of translucent molecular clouds of the Milky Way, suggesting that star formation at high z proceeds in similar environments as today. However, the integrated dust-to-metals ratio is sub-Galactic, suggesting the dust is primarily associated with the molecular phase while the atomic gas has a much lower dust-to-gas ratio. Sightlines like GRB 080607 serve as powerful probes of nucleosynthesis and star-forming regions in the young universe and contribute to the population of "dark" GRB afterglows.

To complement the optical absorption-line survey of diffuse molecular gas in Paper I, we obtained and analyzed far ultraviolet H 2 and CO data on lines of sight toward stars in Cep OB2 and Cep OB3. Possible correlations between column densities of different species for individual velocity components, not total columns along a line of sight as in the past, were examined and were interpreted in terms of cloud structure. The analysis reveals that there are two kinds of CH in diffuse molecular gas: CN-like CH and CH + -like CH. Evidence is provided that CO is also associated with CN in diffuse molecular clouds. Different species are distributed according to gas density in the diffuse molecular gas. Both calcium and potassium may be depleted onto grains in high density gas, but with different dependences on local gas density. Gas densities for components where CN was detected were inferred from a chemical model. Analysis of cloud structure indicates that our data are generally consistent with the large-scale structure suggested by maps of CO millimeter-wave emission. On small scales, the gas density is seen to vary by factors greater than 5.0 over scales of ∼ 10,000 AU. The relationships between column densities of CO and CH with that of H 2 along a line of sight show similar slopes for the gas toward Cep OB2 and OB3, but the CO/H 2 and CH/H 2 ratios tend to differ which we ascribe to variation in average density along the line of sight.

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We examine 20 diffuse and translucent Galactic sight lines and extract the column densities of the 12 CO and 13 CO isotopologues from their ultraviolet AYX absorption bands detected in archival Space Telescope Imaging Spectrograph data with k/Ák ! 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 13 CO. Most sight lines have 12 CO-to-13 CO isotopic ratios that are not significantly different from the local value of 70 for 12 C/ 13 C, which is based on millimeter-wave observations of rotational lines in emission from CO and H 2 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 12 C/ 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 12 CO-to-13 CO ratio with physical conditions such as gas temperature or density, yet 12 CO/ 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 2 show that all sight lines with detected amounts of 13 CO pass through gas that is on average colder by 20 K than the gas without 13 CO. This colder gas is also sampled by CN and C 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 12 CO/ 13 CO.