paper

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

arXiv:2212.09334 · doi:10.1051/0004-6361/202244849

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 MillimÃ©trique (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) $\sim 1 0^{- 8} - 1 0^{- 7}$ ) than in translucent clouds ( $X$ (H $_{2}$ S) and $X$ (CS) $\sim 1 0^{- 9} - 1 0^{- 8}$ ) 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