Bottlenecks To Interstellar Sulfur Chemistry: Sulfur-bearing Hydrides Iin UV-illuminated Gas And Grains

Overview of the Orion Bar. The (000, 000) position corresponds to α2000 = 05h 35m 20.1 s ; δ2000 = − 05◦25007.0 00 . Left panel: Integrated line intensity maps in the 13CO J = 3-2 (color scale) and SO 89-78 emission (gray contours; from 6 to 23.5 K km s−1 in steps of 2.5 K km s−1 ) obtained with the IRAM 30 m telescope at 800 resolution. The white dotted contours delineate the position of the H2 dissociation front as traced by the infrared H2 v = 1–0 S (1) line (from 1.5 to 4.0 × 10−4 erg s−1 cm−2 sr−1 in steps of 0.5 × 10−4 erg s−1 cm−2 sr−1 ; from Walmsley et al. 2000). The black-dashed rectangle shows the smaller FoV imaged with ALMA (Fig. 3). The DF position has been observed with SOFIA, IRAM 30 m, and Herschel. Cyan circles represent the ∼1500 beam at 168 GHz. Right panel: H2S lines lines detected toward three positions of the Orion Bar.

Hydride molecules lie at the base of interstellar chemistry, but the synthesis of sulfuretted hydrides is poorly understood. Motivated by new observations of the Orion Bar PDR - 1'' resolution ALMA images of SH+; IRAM 30m detections of H2S, H2S34, and H2S33; H3S+ (upper limits); and SOFIA observations of SH - we perform a systematic study of the chemistry of S-bearing hydrides.

We determine their column densities using coupled excitation, radiative transfer as well as chemical formation and destruction models. We revise some of the key gas-phase reactions that lead to their chemical synthesis. This includes ab initio quantum calculations of the vibrational-state-dependent reactions SH+ + H2 <-> H2S+ + H and S + H2 <-> SH + H. We find that reactions of UV-pumped H2 (v>1) with S+ explain the presence of SH+ in a high thermal-pressure gas component, P_th~10^8 cm^-3 K, close to the H2 dissociation front.

However, subsequent hydrogen abstraction reactions of SH+, H2S+, and S with vibrationally excited H2, fail to ultimately explain the observed H2S column density (~2.5x10^14 cm^-2, with an ortho-to-para ratio of 2.9+/-0.3). To overcome these bottlenecks, we build PDR models that include a simple network of grain surface reactions leading to the formation of solid H2S (s-H2S).

The higher adsorption binding energies of S and SH suggested by recent studies imply that S atoms adsorb on grains (and form s-H2S) at warmer dust temperatures and closer to the UV-illuminated edges of molecular clouds. Photodesorption and, to a lesser extent, chemical desorption, produce roughly the same H2S column density (a few 10^14 cm-^2) and abundance peak (a few 10^-8) nearly independently of n_H and G_0. This agrees with the observed H2S column density in the Orion Bar as well as at the edges of dark clouds without invoking substantial depletion of elemental sulfur abundances.

J. R. Goicoechea, A. Aguado, S. Cuadrado, O. Roncero, J. Pety, E. Bron, A. Fuente, D. Riquelme, E. Chapillon, C. Herrera, C. A. Duran

Comments: Accepted for publication in A&A, 25 pages (abridged abstract)
Subjects: Astrophysics of Galaxies (astro-ph.GA)
Cite as: arXiv:2101.01012 [astro-ph.GA] (or arXiv:2101.01012v1 [astro-ph.GA] for this version)
Submission history
From: J. R. Goicoechea
[v1] Mon, 4 Jan 2021 14:45:24 UTC (2,993 KB)
Astrochemistry, Astrobiology,

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