Abundances and behavior of (CO)-C-12, (CO)-C-13, and C-2 in translucent sight lines

Using UV spectra obtained with FUSE, HST, and/or IUE together with higher resolution optical spectra, we determine interstellar column densities of (CO)-C-12, (CO)-C-13, and/or C-2 for 10 Galactic sight lines with E(B-V) ranging from 0.37 to 0.72. The N(CO)/N(H-2) ratio varies over a factor of 100 in this sample, due primarily to differences in N(CO). For a given N(H2), published models of diffuse and translucent clouds predict less CO than is observed. The J = 1-3 rotational levels of (CO)-C-12 are subthermally populated in these sight lines, with T-ex typically between 3 and 7 K. In general, there appears to be no significant difference between the excitation temperatures of (CO)-C-12 and (CO)-C-13. Fits to the higher resolution CO line profiles suggest that CO (like CN) is concentrated in relatively cold, dense gas. We obtain C-2 column densities from the F-X (0-0) band at 1341 angstrom (three sight lines; J = 0-12), the F-X (1-0) band at 1314 angstrom (one sight line; J = 0-12), the D-X (0-0) band at 2313 angstrom (four sight lines; J = 0-18), and the A-X (3-0) and (2-0) bands at 7719 and 8757 angstrom (seven sight lines; J = 0-12). Comparisons among those column densities yield a set of mutually consistent band f-values for the UV and optical C-2 bands, but also reveal some apparent anomalies within the F-X (0-0) band. Both the kinetic temperature T-k inferred from the C-2 rotational populations and the excitation temperature T-02(C-2) are generally smaller than the corresponding T-01(H-2) - suggesting that C-2 is concentrated in colder, denser gas than H2. Incorporating additional column density data for K 1, HD, CH, C-2, C-3, CN, and CO from the literature (for a total sample of 74 sight lines), we find that (1) COis most tightly correlated with CN; (2) the ratios (CO)-C-12/H-2 and (CO)-C-13/H-2 both are fairly tightly correlated with the density indicator CN/CH (but C-2/H-2 is not); and (3) the ratio (CO)-C-12/(CO)-C-13 is somewhat anticorrelated with both CN/CH and N(CO). Sight lines with (CO)-C-12/(CO)-C-13 below the average local Galactic value of C-12/C-13 appear to sample colder, denser gas in which isotope exchange reactions have enhanced (CO)-C-13, relative to (CO)-C-12.