Photoprocessing of H₂S on dust grains

Abstract

Context. Sulfur is a biogenic element used as a tracer of the evolution of interstellar clouds to stellar systems. However, most of the expected sulfur in molecular clouds remains undetected. Sulfur disappears from the gas phase in two steps. The first depletion occurs during the translucent phase, reducing the gas-phase sulfur by 7–40 times, while the following freeze-out step occurs in molecular clouds, reducing it by another order of magnitude. This long-standing question awaits an explanation. Aims. The aim of this study is to understand under what form the missing sulfur is hiding in molecular clouds. The possibility that sulfur is depleted onto dust grains is considered. Methods. Experimental simulations mimicking H2S ice UV photoprocessing in molecular clouds were conducted at 8 K under ultra-high vacuum. The ice was subsequently warmed up to room temperature. The ice was monitored using infrared spectroscopy, and the desorbing molecules were measured by quadrupole mass spectrometry in the gas phase. Theoretical Monte Carlo simulations were performed for interpretation of the experimental results and extrapolation to the astrophysical and planetary conditions. Results. H2S2 formation was observed during irradiation at 8 K. Molecules H2Sx with x > 2 were also identified and found to desorb during warm-up, along with S2 to S4 species. Larger Sx molecules up to S8 are refractory at room temperature and remained on the substrate forming a residue. Monte Carlo simulations were able to reproduce the molecules desorbing during warming up, and found that residues are chains of sulfur consisting of 6–7 atoms. Conclusions. Based on the interpretation of the experimental results using our theoretical model, it is proposed that S+ in translucent clouds contributes notoriously to S depletion in denser regions by forming long S chains on dust grains in a few times 104 yr. We suggest that the S2 to S4 molecules observed in comets are not produced by fragmentation of these large chains. Instead, they probably come either from UV photoprocessing of H2S-bearing ice produced in molecular clouds or from short S chains formed during the translucent cloud phase.

The sulphur depletion problem in molecular clouds: The H2S case

“…We first notice the presence of H 2 S in the gas phase at the most external positions where CH 3 OH is not detected. This is compatible with an early depletion of Sulphur atoms onto grain surfaces [7], before significant depletion of CO. Since reactions with CO are the main destruction path of N 2 H + [8], when CO depletion occurs, N 2 H + starts increasing its abundance.…”

AB Aur, a Rosetta stone for studies of planet formation

“…We highlight, however, that H 2 S is the most abundant sulfuretted species in comets (Bockelée-Morvan & Biver 2017), in agreement with our results. Cazaux et al (2022) find that, under efficient self-shielding conditions, H 2 S survives on the surface of grains as the ice below 100 K, and is largely depleted from the grains at 150 K. However, if no self-shielding is assumed, most H 2 S is transformed, under the influence of UV radiation, into S x sulfur chains. Our Nautilus model predicts that gaseous S 8 and that the surface of grains is the seventh and eighth major sulfur carrier.…”

Gas phase Elemental abundances in Molecular cloudS (GEMS)

“…According to these measurements, the abundances of the icy species could account for <5% of the total sulphur. It has been suggested that this socalled depleted sulphur may be locked as refractory material (in particular as S 8 , Shingledecker et al 2020;Cazaux et al 2022), claiming sulphur depletion of more than two orders of magnitude. Intense observational and theoretical work has been carried out in the last decade to constrain the value of sulphur depletion, and there is increasing evidence for a moderate factor of 1-20 of sulphur depletion in cold and warm clouds (Esplugues et al 2013(Esplugues et al , 2014Fuente et al 2019;Navarro-Almaida et al 2020Bulut et al 2021;Codella et al 2021).…”

“…However, H 2 S has never been detected in interstellar ices, and its abundance has been estimated to be smaller than 5 × 10 −8 (/H) (Jiménez-Escobar & Muñoz Caro 2011). Other solid species have been proposed as possible sulphur reservoirs, such as OCS, SO 2 , H 2 S 2 , CS 2 , and S 8 (e.g., Palumbo et al 1997;Druard & Wakelam 2012;Laas & Caselli 2019;Shingledecker et al 2020;Cazaux et al 2022), OCS being the only S-bearing molecule unambiguously detected in ice mantles in the infrared (Geballe et al 1985;Palumbo et al 1995) along with, tentatively, SO 2 (Boogert et al 1997;Zasowski et al 2009).…”