Ice origins of OCS Rafael Martín-Doménech 1,2 , Karin I. Öberg 2 , Guillermo M. Muñoz Caro 1
1 Centro de Astrobiología (CSIC-INTA). Carretera de Ajalvir, Km. 4, Torrejón de Ardoz, E-28850, Madrid, Spain. 2 Center for Astrophysics | Harvard & Smithsonian. 60 Garden St., Cambridge, MA 02138, USA The fate of sulfur in the interior of dense interstellar clouds is currently uncertain, and the fraction of sulfur present in the gas phase, ice mantles, and refractories is unconstrained [1]. As a result, the final form in which volatile sulfur is incorporated into star-forming regions and could eventually participate in prebiotic chemical networks is unknown. Complex organic and prebiotic chemistry takes place upon energetic processing of interstellar ice mantles, so the fraction of sulfur present in such ices is of particular interest. The only S-bearing ice molecule confirmed to date is OCS [2,3], while SO 2 detection remains tentative [3]. Recent observations with the NASA InfRared Telescope Facility and James Webb Space Telescope have notably increased the number of sources in which OCS ice is detected [2,3]. Understanding how the initial reservoir of sulfur in interstellar ices is built is of vital importance to understand the sulfur chemistry. Due to the low gas-phase OCS abundance, in situ ice formation is required [2]. Solid-state formation pathways suggested in the literature include oxidation of CS (CS+O) or sulfurization of CO (CO+S) through non-energetic or energetic processes [2]. The CS+O pathway has been previously studied in the laboratory through thermal reaction of CS 2 with O atoms [4], and electron irradiation of CS 2 :O 2 ices [5]. The CO+S pathway has been explored through proton and UV-photon irradiation of CO/CO 2 ices with SO 2 or H 2 S as the sulfur source [6-8]. We have expanded on these works and studied the OCS formation upon electron irradiation of CO:CS 2 and H 2 O:CS 2 ice analogs at 6 K. Formation of OCS was detected in both cases using IR spectroscopy and quadrupole mass spectrometry. We used isotopically-labeled CO in CO:CS 2 mixtures that allowed us to determine that OCS formation proceeded through both the CS+O and CO+S pathways. In H 2 O:CS 2 ice samples, OCS formation took place to a similar extent via the CS+O pathway, but any additional sulfur chemistry was significantly quenched. We have also evaluated the formation of OCS in a CH 4 :SO 2 ice where the CO and CS moieties were not initially present. In this case, the chemistry was hydrocarbon-dominated, and formation of OCS was not detected. References 1. Laas, J. & Caselli, P. 2019, A&A, 624, A108
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