2008ApJ...684.1210F

Extensive experimental studies have been performed on the solid-state formation of the OCS molecule in proton-irradiated water-free and water-dominated ices containing CO or CO₂ as the carbon source and H₂S or SO₂as the sulfur source. In each case OCS is readily formed. Production efficiency follows the trends CO>CO₂ and H₂S>SO₂as C,O- and S-sources, respectively. In water-dominated ices, OCS production appears to be enhanced for CO:H₂S reactants. The mechanism of formation of OCS appears to be the reaction of CO with free S atoms produced by fragmentation of the sulfur parent species. While OCS is readily formed by irradiation, it is also the most easily destroyed on continued exposure. In H₂O-dominated ices the half-life of H₂S, SO₂, and OCS is ∼2 eV/molecule, corresponding to ∼7 million years in a cold dense interstellar cloud environment processed by cosmic-ray protons. The spectral profile of the ν₃ band of OCS is highly dependent on temperature and ice composition, and changes with radiation processing. These effects can be used in theoretical modeling of interstellar infrared (IR) spectra; a laboratory spectrum of irradiated H₂O:CO:H₂S, warmed to 50 K, provides a good fit to the 2040/cm feature in the W33A spectrum. The identification of OCS in CO₂-dominated ices provides a further challenge, due to the overlap of the OCS band with that of CO₃ formed from irradiation of the host ice. The two features can be unraveled by a curve-fitting procedure. It is the width of the 2040/cm band that will help observers determine if features identified in CO₂-rich ices are due to OCS or to CO₃.