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Observations of interstellar dust are often used as a proxy for total gas column density N H. By comparing Planck thermal dust data (Release 1.2) and new dust reddening maps from Pan-STARRS 1 and 2MASS, with accurate (opacity-corrected) H i column densities and newly published OH data from the Arecibo Millennium survey and 21-SPONGE, we confirm linear correlations between dust optical depth τ 353, reddening E(B - V), and the total proton column density N H in the range (1-30) × 1020 cm-2, along sightlines with no molecular gas detections in emission. We derive an N H/E(B - V) ratio of (9.4 ± 1.6) × 1021 cm-2 mag-1 for purely atomic sightlines at , which is 60% higher than the canonical value of Bohlin et al. We report a ∼40% increase in opacity σ 353 = τ 353/N H, when moving from the low column density (N H < 5 × 1020 cm-2) to the moderate column density (N H > 5 × 1020 cm-2) regime, and suggest that this rise is due to the evolution of dust grains in the atomic interstellar medium. Failure to account for H i opacity can cause an additional apparent rise in σ 353 of the order of a further ∼20%. We estimate molecular hydrogen column densities from our derived linear relations, and hence derive the OH/H2 abundance ratio of X OH ∼ 1 × 10-7 for all molecular sightlines. Our results show no evidence of systematic trends in OH abundance with in the range ∼ (0.1-10) × 1021 cm-2. This suggests that OH may be used as a reliable proxy for H2 in this range, which includes sightlines with both CO-dark and CO-bright gas.
- ISM: clouds
- ISM: molecules
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