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We present an observational study of the sulfur (S)-bearing species toward Orion KL at 1.3 mm by combining ALMA and IRAM-30 m single-dish data. At a linear resolution of ∼800 au and a velocity resolution of 1 km s−1, we have identified 79 molecular lines from six S-bearing species. In these S-bearing species, we found a clear dichotomy between carbon–sulfur compounds and carbon-free S-bearing species for various characteristics, e.g., of line profiles, spatial morphology, and molecular abundances with respect to H2. Lines from the carbon–sulfur compounds (i.e., OCS, 13CS, and H2CS) exhibit spatial distributions concentrated around the continuum peaks and extended to the south ridge. The full width at half maximum (FWHM) linewidth of these molecular lines is in the range of 2 ∼11 km s−1. The molecular abundances of OCS and H2CS decrease slightly from the cold (∼68 K) to the hot (∼176 K) regions. In contrast, lines from the carbon-free S-bearing species (i.e., SO2, 34SO, and H2S) are spatially more extended to the northeast of mm4, exhibiting broader FWHM line widths (15 ∼ 26 km s−1). The molecular abundances of carbon-free S-bearing species increase by over an order of magnitude as the temperature increase from 50 to 100 K. In particular, 34SO/34SO2 and OCS/SO2 are enhanced from the warmer regions (>100 K) to the colder regions (∼50 K). Such enhancements are consistent with the transformation of SO2 at warmer regions and the influence of shocks. [ABSTRACT FROM AUTHOR]

Observations of interstellar dust are often used as a proxy for total gas column density NH. 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 τ353, reddening E(B − V), and the total proton column density NH in the range (1–30) × 1020 cm−2, along sightlines with no molecular gas detections in emission. We derive an NH/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 σ353 = τ353/NH, when moving from the low column density (NH < 5 × 1020 cm−2) to the moderate column density (NH > 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 σ353 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 XOH ∼ 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. [ABSTRACT FROM AUTHOR]

We have obtained OH spectra of four transitions in the ground state, at 1612, 1665, 1667, and 1720 MHz, toward 51 sightlines that were observed in the Herschel project Galactic Observations of Terahertz C+. The observations cover the longitude range of (32°, 64°) and (189°, 207°) in the northern Galactic plane. All of the diffuse OH emissions conform to the so-called “Sum Rule” of the four brightness temperatures, indicating optically thin emission conditions for OH from diffuse clouds in the Galactic plane. The column densities of the H i “halos” N(H i) surrounding molecular clouds increase monotonically with OH column density, N(OH), until saturating when cm−2 and cm−2, indicating the presence of molecular gas that cannot be traced by H i. Such a linear correlation, albeit weak, is suggestive of halos’ contribution to the UV shielding required for molecular formation. About 18% of OH clouds have no associated CO emission (CO-dark) at a sensitivity of 0.07 K, but are associated with C+ emission. A weak correlation exists between C+ intensity and OH column density for CO-dark molecular clouds. These results imply that OH seems to be a better tracer of molecular gas than CO in diffuse molecular regions. [ABSTRACT FROM AUTHOR]