2007ApJS..168...58S

Using UV spectra obtained with FUSE, HST, and/or IUE together with higher resolution optical spectra, we determine interstellar column densities of ¹²CO, ¹³CO, and/or C₂for 10 Galactic sight lines with E(B-V) ranging from 0.37 to 0.72. The N(CO)/N(H₂) 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 ¹²CO 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 ¹²CO and ¹³CO. Fits to the higher resolution CO line profiles suggest that CO (like CN) is concentrated in relatively cold, dense gas. We obtain C₂column densities from the F-X (0-0) band at 1341 Å (three sight lines; J=0-12), the F-X (1-0) band at 1314 Å (one sight line; J=0-12), the D-X (0-0) band at 2313 Å (four sight lines; J=0-18), and the A-X (3-0) and (2-0) bands at 7719 and 8757 Å (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 C₂bands, but also reveal some apparent anomalies within the F-X (0-0) band. Both the kinetic temperature T_k inferred from the C₂rotational populations and the excitation temperature T₀₂(C₂) are generally smaller than the corresponding T₀₁(H₂)–suggesting that C₂is concentrated in colder, denser gas than H₂. Incorporating additional column density data for K I, HD, CH, C₂, C₃, CN, and CO from the literature (for a total sample of 74 sight lines), we find that (1) CO is most tightly correlated with CN; (2) the ratios ¹²CO/H₂and ¹³CO/H₂ both are fairly tightly correlated with the density indicator CN/CH (but C₂/H₂is not); and (3) the ratio ¹²CO/¹³CO is somewhat anticorrelated with both CN/CH and N(CO). Sight lines with ¹²CO/¹³CO below the average local Galactic value of ¹²C/¹³C appear to sample colder, denser gas in which isotope exchange reactions have enhanced ¹³CO, relative to ¹²CO.