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Astrophysics Research and Analysis Program

Abundances and Reactions of Sulfur-Containing Molecules in Interstellar Ices – Spectroscopic and Radiation-Chemical Investigations

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Abundances and Reactions of Sulfur-Containing Molecules in Interstellar Ices – Spectroscopic and Radiation-Chemical Investigations
Infrared (IR) telescopes, such as ISO, Spitzer, KAO, Keck, VLT, and IRTF, have revealed a rich variety of molecules trapped in interstellar ices. However, quantifying the abundances of these molecules has been difficult because reference IR data, such as band strengths and optical constants, often are poorly known. This scarcity of data has severe implications for the study of sulfur-containing molecules, such as OCS and SO2, since accurate molecular abundances are needed to address the missing-sulfur problem in interstellar space. The expected abundances of sulfur-containing species in dense molecular clouds are much higher than reported from telescopic observations, although the latter are based on laboratory data of questionable relevance, such as with liquids at room temperature compared to the 10 K ices of some interstellar regions. Exacerbating the problem is that few sulfur-containing molecules of any type have been examined in the laboratory under the necessary, relevant icy conditions. We propose to address and correct the problems associated with abundance determinations of interstellar sulfur-containing ices. We will combine several recent successful efforts from our laboratory and measure near- and mid-IR spectral intensities for ices containing SO2, OCS, H2S, CS2, CH3SH, and C2H5SH both in the presence and absence of H2O-ice. This work will be done at multiple temperatures and ice phases to generate reference IR spectra and band strengths, accompanied by refractive indices, and optical constants. Moreover, we will study the radiation chemistry of these molecules to determine their radiolytic half-lives (stabilities) and uncover product molecules that can become candidates for future searches and perhaps help better understand the missing-sulfur problem. This proposal is a convergence of three lines of work in our laboratory: recent successes in deriving IR optical constants of interstellar molecular ices (Hudson et al. 2014a, 2014b), measurements of radiolytic stabilities of interstellar and planetary molecules (Gerakines & Hudson 2013, 2015), and our long history of working with sulfur-containing molecules and ions (Moore et al. 1988; Loeffler & Hudson 2010, 2012). Our collaborators were selected specifically for their interest in this proposal's focus and for their expertise in interstellar chemistry. More »

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