Formation of carbonyl sulfide (OCS) via SH radicals in interstellar CO-rich ice under dense cloud conditions

Laboratory for Astrophysics, Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands

Received: 3 July 2024
Accepted: 6 August 2024

Abstract

Context. Carbonyl sulfide (OCS) is widely observed in the gas phase toward star-forming regions and was the first of the only two sulfur-bearing species to be detected in interstellar ices so far. However, the chemical network governing its formation is still not fully understood. While the sulfurization of CO and the oxidation of CS are often invoked to form OCS, other mechanisms could have a significant contribution. In particular, the multistep reaction involving CO and SH is a good candidate for forming a significant portion of the OCS in dense cloud environments.

Aims. We aim to constrain the viability of the CO + SH route for forming solid OCS in the interstellar medium, in a similar manner as CO + OH is known to produce CO₂ ice. This is achieved by conducting a systematic laboratory investigation of the targeted reactions on interstellar ice analogs under dense cloud conditions.

Methods. We used an ultrahigh vacuum chamber to simultaneously deposit CO, H₂S, and atomic H at 10 K. SH radicals produced in situ via hydrogen abstraction from H₂S reacted with CO to form OCS. We monitored the ice composition during deposition and subsequent warm-up by means of Fourier-transform reflection absorption infrared spectroscopy (RAIRS). Complementarily, desorbed species were recorded with a quadrupole mass spectrometer (QMS) during temperature-programmed desorption (TPD) experiments. Control experiments were performed to secure the product identification. We also explored the effect of different H₂S:CO mixing ratios – with decreasing H₂S concentrations – on the OCS formation yield.

Results. OCS is efficiently formed through surface reactions involving CO, H₂S, and H atoms. The suggested underlying mechanism behind OCS formation is CO + SH → HSCO, followed by HSCO + H → OCS + H₂. The OCS yield reduces slowly, but remains significant with increasing CO:H₂S mixing ratios (CO:H₂S = 1:1, 5:1, 10:1, and 20:1).

Conclusions. Our experiments provide unambiguous evidence that OCS can be formed from CO + SH in the presence of H atoms. This route remains efficient for large H₂S dilutions (5% with respect to CO), suggesting that it is a viable mechanism in interstellar ices. Given that SH radicals can be created in clouds over a wide evolutionary timescale, this mechanism could make a non-negligible contribution to the formation of interstellar OCS ice.