Ice origins of OCS and chemistry of CS2-bearing ice mantles

TL;DR

This study investigates the formation pathways of OCS in interstellar ices, finding that oxidation of CS is more favorable than sulfurization of CO, especially at higher temperatures, and explores sulfur chemistry in various ice compositions.

Contribution

It provides experimental evidence favoring the CS+O pathway for OCS formation and explores sulfur chemistry in ices relevant to the interstellar medium.

Findings

OCS formation via CS+O pathway is more favorable.

Higher temperatures increase the contribution of the CS+O pathway.

Various sulfur-bearing molecules, including SO2, C3S2, and S2, are formed.

Abstract

Understanding the formation of carbonyl sulfide (OCS) in interstellar ices is key to constrain the sulfur chemistry in the interstellar medium (ISM), since it is the only ice S-bearing molecule securely detected thus far. Two general pathways for OCS formation have been proposed: sulfurization of CO (CO+S) and oxidation of CS (CS+O), but their relative contribution in interstellar ices remains unconstrained. We have evaluated the contribution of both pathways to OCS formation upon energetic processing in isotopically-labeled CO2:CS2 and CO:CS2 ice samples at 7-50 K. Our results indicated that formation of OCS through the CS+O pathway was more favorable than through the CO+S pathway, as previously suggested by theoretical calculations. In addition, its relative contribution increased at higher temperatures. Therefore, this pathway could play a role in the ice formation of OCS, especially in warm regions where CO is expected to be preferentially in the gas phase. At the same time, we have explored the chemistry of CS2-bearing, CO2-, CO-, and also H2O-rich ices, that could be relevant to the sulfur interstellar chemistry. We observed formation of a variety of S-bearing products in addition to OCS, including SO2, C3S2, and S2. However, a significant fraction of sulfur was not detected at the end of the experiments, and could be locked in long, undetectable sulfur allotropes, one of the potential carriers of the missing sulfur in the dense ISM.