Bottlenecks to interstellar sulfur chemistry

Abstract

Hydride molecules lie at the base of interstellar chemistry, but the synthesis of sulfuretted hydrides is poorly understood and their abundances often crudely constrained. Motivated by new observations of the Orion Bar photodissociation region (PDR) – 1″ resolution ALMA images of SH+; IRAM 30 m detections of bright H232S, H234S, and H233S lines; H3S+ (upper limits); and SOFIA/GREAT observations of SH (upper limits) – we perform a systematic study of the chemistry of sulfur-bearing hydrides. We self-consistently 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(v) ⇄ H2S+ + H and S + H2 (v) ⇄ SH + H. We find that reactions of UV-pumped H2(v ≥ 2) molecules with S+ ions explain the presence of SH+ in a high thermal-pressure gas component, Pth∕k ≈ 108 cm−3 K, close to the H2 dissociation front (at AV < 2 mag). These PDR layers are characterized by no or very little depletion of elemental sulfur from the gas. However, subsequent hydrogen abstraction reactions of SH+, H2S+, and S atoms with vibrationally excited H2, fail to form enough H2S+, H3S+, and SH to ultimately explain the observed H2S column density (~2.5 × 1014 cm−2, with an ortho-to-para ratio of 2.9 ± 0.3; consistent with the high-temperature statistical value). 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 (Td < 50 K) and closer to the UV-illuminated edges of molecular clouds. We show that everywhere s-H2S mantles form(ed), gas-phase H2S emission lines will be detectable. Photodesorption and, to a lesser extent, chemical desorption, produce roughly the same H2S column density (a few 1014 cm−2) and abundance peak (a few 10−8) nearly independently of nH and G0. 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.

The initial gas-phase sulfur abundance in the Orion Molecular Cloud from sulfur radio recombination lines

“…This emission traces the C + layers of the molecular cloud. Cyan contours show a map of the SH + 1 0 -0 1 F = 1/2-3/2 emission at 345.944 GHz (Goicoechea et al 2021), a proxy of the S + layer (both maps at θ HPBW 20 ). Black contours show the O i 1.317 µm fluorescent emission that delineates the the IF of the Bar (Walmsley et al 2000).…”

“…Although S has a lower IP than C, the extent of the S + layer depends on the wavelength-and A V -dependent attenuation of the FUV field (Goicoechea & Le Bourlot 2007) as well as on details of the S photoionization and S + radiative recombination (RR) and resonant dielectronic recombination (DR) processes. Paradoxically, a modern treatment of S + recombination, especially at low T e , is still lacking (Badnell 1991;Bryans et al 2009).…”

“…1 shows that the observed DF position coincides with the position of the SH + emission peak (cyan contours). This trace molecular ion forms via gas reactions of S + with vibrationally excited H 2 molecules (e.g., Goicoechea et al 2021).…”

“…Sulfur plays an important role in stellar nucleosynthesis; it is mainly produced in massive stars (e.g., Perdigon et al 2021), in star formation, and in astrochemistry (e.g., Fuente et al 2017;Shingledecker et al 2020). Inside star-forming clouds, an undetermined fraction of sulfur gradually converts into molecules and ice mantles (e.g., Goicoechea et al 2021), and yet the initial [S] in these dense molecular clouds is poorly constrained.…”

“…This model is valid for the outer C + layer and reproduces the observed IF-A V (CO) angular separation. The chemical network includes updated gas-phase and grain-surface reactions(Goicoechea et al 2021).…”