HyGAL: Characterizing the Galactic ISM with observations of hydrides and other small molecules II. The absorption line survey with the IRAM 30 m telescope

Abstract

As a complement to the HyGAL Stratospheric Observatory for Infrared Astronomy Legacy Program, we report the results of a ground-based absorption line survey of simple molecules in diffuse and translucent Galactic clouds. Using the Institut de Radioastronomie Millimétrique (IRAM) 30 m telescope, we surveyed molecular lines in the 2 mm and 3 mm wavelength ranges toward 15 millimeter continuum sources. These sources, which are all massive star-forming regions located mainly in the first and second quadrants of the Milky Way, form the subset of the HyGAL sample that can be observed by the IRAM 30 m telescope. We detected HCO${}^{+}$ absorption lines toward 14 sightlines, toward which we identified 78 foreground cloud components, as well as lines from HCN, HNC, C${}_2$H, and c-C${}_3$H${}_2$ toward most sightlines. In addition, CS and H${}_2$S absorption lines are found toward at least half of the continuum sources. Static Meudon photodissociation region (PDR) isobaric models that consider ultraviolet-dominated chemistry were unable to reproduce the column densities of all seven molecular species by just a factor of a few, except for H${}_2$S. The inclusion of other formation routes driven by turbulent dissipation could possibly explain the observed high column densities of these species in diffuse clouds. There is a tentative trend for H${}_2$S and CS abundances relative to H${}_2$ to be larger in diffuse clouds ($X$(H${}_2$S) and $X$(CS) $\sim 10^{-8}-10^{-7}$) than in translucent clouds ($X$(H${}_2$S) and $X$(CS) $\sim 10^{-9}-10^{-8}$) toward a small sample; however, a larger sample is required in order to confirm this trend. The derived H${}_2$S column densities are higher than the values predicted from the isobaric PDR models, suggesting that chemical desorption of H${}_2$S from sulfur-containing ice mantles may play a role in increasing the H${}_2$S abundance.

41 pages, 25 figures, 10 tables, and 5 appendices. Accepted for publication in A&A

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