Abundances and Behavior of 12 CO, 13 CO, and C 2 in Translucent Sight Lines

Abstract

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 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(H_2 ), 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 C_2 column 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 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 H_2. Incorporating additional column density data for K I, HD, CH, C_2, C_3, 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/H_2 and ^13CO/H_2 both are fairly tightly correlated with the density indicator CN/CH (but C_2/H_ 2 is 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. Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the Data Archive at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. Based in part on observations obtained with the Apache Point Observatory 3.5 m telescope, which is owned and operated by the Astrophysical Research Consortium.