Atomic and Molecular Carbon as a Tracer of Translucent Clouds
  Using archival, high-resolution far-ultraviolet HST/STIS spectra of 34
Galactic O and B stars, we measure CI column densities and compare them with
measurements from the literature of CO and H_2 with regard to understanding the
presence of translucent clouds along the line-of-sight. We find that the CO/H_2
and CO/CI ratios provide good discriminators for the presence of translucent
material, and both increase as a function of molecular fraction, f =
2N(H_2)/N(H). We suggest that sightlines with values below CO/H_2 ~ 1E-6 and
CO/CI ~ 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 v.
H_2 correlation near the column density at which CO shielding becomes
important, as evidenced by the change in photochemistry regime studied by
Sheffer et al. (2008). 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.
