HyGAL: Characterizing the Galactic ISM with observations of hydrides and other small molecules

Abstract

As a complement to the HyGAL Stratospheric Observatory for Infrared Astronomy Legacy Program, we report the results of a groundbased 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, C2H, and c-C3H2 toward most sightlines. In addition, CS and H2S absorption lines are found toward at least half of the continuum sources. The spectral line data obtained were analyzed to characterize the chemical and physical properties of the absorbing interstellar medium statistically. The column density ratios of the seven molecular species observed are very similar to values found in previous absorption line studies carried out toward diffuse clouds at high latitudes. As expected, the C2H and c-C3H2 column densities show a tight correlation with that of N(HCO+), because of these all these molecules are considered to be proxies for the H2 column density toward diffuse and translucent clouds. The HCN and HNC column densities, by contrast, exhibit nonlinear correlations with those of C2H, c-C3H2, and HCO+, increasing rapidly at Av ≈ 1 in translucent clouds. 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 H2S. 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 H2S and CS abundances relative to H2 to be larger in diffuse clouds (X(H2S) and X(CS) ~ 10−8−10−7) than in translucent clouds (X(H2S) and X(CS) ~ 10−9−10−8) toward a small sample; however, a larger sample is required in order to confirm this trend. The derived H2S column densities are higher than the values predicted from the isobaric PDR models, suggesting that chemical desorption of H2S from sulfur-containing ice mantles may play a role in increasing the H2S abundance.