2010ApJ...708..334B

Using archival, high-resolution far-ultraviolet Hubble Space Telescope/Space Telescope Imaging Spectrograph spectra of 34 Galactic O and B stars, we measure C I column densities and compare them with measurements from the literature of CO and H₂ with regard to understanding the presence of translucent clouds along the line of sight. We find that the CO/H₂ and CO/C I ratios provide good discriminators for the presence of translucent material, and both increase as a function of molecular fraction, f^N = 2N(H₂)/N(H). We suggest that sightlines with values below CO/H₂~10^–6 and CO/C I ~1 contain mostly diffuse molecular clouds, while those with values above sample clouds in the transition region between diffuse and dark. These discriminating values are also consistent with the change in slope of the CO versus H₂ correlation near the column density at which CO shielding becomes important, as evidenced by the change in photochemistry regime studied by Sheffer et al. Based on the lack of correlation of the presence of translucent material with traditional measures of extinction, we recommend defining "translucent clouds" based on the molecular content rather than line-of-sight extinction properties.