Formation of OCS in interstellar ices

The high sensitivity and spectral resolution offered by the JWST have allowed us to expand the search for new species, both in the gas and the solid phases. The S-bearing species are of particular interest due to its relevance in the star-formation process (Fuente et al. 2023) and the origin of life (Leustek 2002, Francioso et al. 2020). Unfortunately, current observations cannot account for the expected sulfur abundance in star-forming regions (Laas & Caselli 2019). The missing sulfur is expected to be locked in the solid phase (Millar & Herbst 1990). However, JWST observations of ice mantles in dense molecular clouds have only been able to detect OCS (McClure et al. 2023) and, tentatively, SO2 (Rocha et al. 2024), with abundances that only account for <5% of the cosmic sulfur abundance. Understanding how this reservoir of sulfur in interstellar ices is built is of vital importance to constrain the evolution of the sulfur chemistry during star-formation. In the case of OCS, in situ formation on the grain surface through the CO+S and/or CS+O pathways needs to be invoked (Boogert et al. 2022). We present laboratory experiments comparing the relative contribution of these two pathways to the OCS formation in interstellar ices. We found that the CS+O pathway is 3-6 times more efficient, depending on the ice temperature. In addition, a significant fraction of the initial S in our experiments seemed to form sulfur chains, that could contain part of the missing sulfur in the ISM.