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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 H₂S_x with x > 2 were also identifyed and found to desorb during warm-up, along with S₂ to S₄ species. Larger S_x 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.