Laboratory Infrared Spectra and Band Strengths of Carbonyl Sulfide (OCS) in CH3OH- and CO-Rich Ice Mixtures for Analyzing Interstellar Ice Observations

Carbonyl sulfide (OCS) is currently the only securely detected sulfur-bearing species in interstellar ices, making it an ideal window into solid-state sulfur chemistry in dense star-forming regions. Previous astronomical observations of the OCS asymmetric stretching mode (ν₃) at ∼2040 cm^–1 (∼4.9 μm) demonstrate that interstellar OCS may be embedded in CH₃OH-rich ices, indicating that OCS likely forms in the coldest, densest parts of star-forming regions where catastrophic CO freezeout occurs. However, a significant portion of the OCS ice observations cannot be fit with binary OCS:CH₃OH laboratory ice mixtures alone, suggesting a greater degree of chemical complexity in the local ice environment. With this work, we aim to aid future studies of the abundance, physicochemical environment, and evolutionary history of interstellar OCS ice, now enabled for many more interstellar environments by the James Webb Space Telescope. We provide a library of new laboratory IR transmission spectra of the tetrahedron of the OCS in CH₃OH- and CO-rich ice mixtures, some of which also include H₂S and H₂O. Of these new spectra, the tertiary OCS:CO:CH₃OH ice mixtures provide the best fits to observations of high-mass protostars, providing further support for the hypothesis that the atom of the OCS forms with CH₃OH, possibly via chemical pathways involving frozen-out CO. We calculate apparent band strengths of the ν₃ mode in the OCS:CH₃OH and the OCS:CO:CH₃OH ice mixtures. The derived values are consistent (within uncertainties) with the apparent band strength of the feature in pure OCS ice, 1.2 × 10^–16 cm molec^–1. We therefore recommend using this value when quantifying interstellar OCS ice column densities.