Atomic and Molecular Carbon as a Tracer of Translucent Clouds

TL;DR

This study uses ultraviolet spectra to identify molecular ratios that effectively distinguish translucent clouds from diffuse clouds, proposing molecular content as a better classification criterion than extinction measures.

Contribution

It introduces specific molecular ratio thresholds (CO/H_2 and CO/CI) as indicators for translucent clouds, advancing cloud classification methods.

Findings

CO/H_2 and CO/CI ratios increase with molecular fraction.

Thresholds of CO/H_2 ~ 1E-6 and CO/CI ~ 1 differentiate cloud types.

Molecular content is a more reliable indicator than extinction for cloud classification.

Abstract

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.