Sulphur depletion in dense clouds and circumstellar regions. Organic products made from UV photoprocessing of realistic ice analogs containing H2S

In a recent article, we reported the UV irradiation of H2S:H2O ice and estimated upper limits for solid H2S in protostars. Photoproducts of H2S ice processing were proposed as a plausible explanation of sulphur depletion towards dense clouds and protostars. But the photochemistry of H2S with C-bearing molecules like CO and CH3OH was not explored. We simulate experimentally the formation of organic products made from UV irradiation and warm-up of CO or CH3OH ice containing H2S. Our experiments were performed under ultrahigh vacuum conditions using the Interstellar Astrochemistry Chamber. We used infrared spectroscopy and quadrupole mass spectrometry to monitor the solid and gas phase simultaneously during the experiments. The main species produced after irradiation of H2S:CO ice mixtures were H2S2, HS2, CS2, H2CO, and OCS. In H2S:CH3OH irradiation experiments CO, CO2, CH4, H2CO, and CS2 were formed. A complex organic refractory residue remained at room temperature. This experimental evidence supports that dust grains are likely a reservoir of the missing sulphur in dense clouds and circumstellar regions. Indeed, with the exception of CS2 and OCS, we found that most of the sulphur contained in refractory photoproducts that were stable at room temperature is likely in the form of S-chains. Our results are used to interpret the observations of OCS and CS2 in different astrophysical environments.