Origin and fate of interstellar S-bearing molecules: insights from observations and experiments

The observed sulfur content in dense interstellar clouds and protostellar environments constitutes only a small fraction of the expected cosmic value, with the bulk of its reservoir remaining unknown. One hypothesis is that the missing sulfur is locked away in or beneath the icy mantles that shroud interstellar dust grains, challenging its observation. This distinctive feature of sulfur-bearing species makes them critical for understanding the evolution of volatile and refractory components during star and planet formation. In this presentation, I will showcase recent laboratory experiments on interstellar ice analogues that offer particularly promising new pathways to S-bearing organics. We find that SH radicals can initiate a prolific sulfur reaction network under interstellar conditions, leading to the formation of CH3CH2SH, CH2CHSH, HSCH2CH2SH, H2S2, OCS, and tentatively CH3CHS and CH2CS. Computational calculations further elucidate key reaction routes in this network. I will also present ALMA observations of gaseous SO2 and OCS towards 26 MYSOs from the ALMAGAL survey, compared with literature ice data—including JWST observations. These species are particularly relevant since they are major carriers of gaseous sulfur and the only sulfurated molecules detected in ices to date. Such comparisons provide powerful information on the chemical and physical environments of these molecules and their potential inheritance by planetesimals.