"The Evolution of Astrophysical Ices: The Carbon Dioxide Diagnosis"
We have used the Infrared Spectrometer on board the Spitzer Space Telescope to carry out a comprehensive study of the carbon dioxide bending mode absorption feature centered near 15 micrometers in astrophysical ices. Previous observations with the Infrared Space Observatory, together with studies of laboratory analogs, have shown that this feature has strong diagnostic properties. Substructures within the feature are sensitive to the thermal history of the ices and to the formation of linked carbon dioxide/methanol complexes. Both of these molecules are important repositories for carbon in interstellar ices, and their roles in the chemical evolution of the ices and their sublimation products are intimately linked. The abundance of carbon dioxide relative to methanol is diagnostic of key reaction pathways, measuring the relative efficiencies of catalytic oxidation and hydrogenation reactions in cold dark clouds. In regions exposed to the interstellar radiation field, photolytic reactions may contribute to their formation. The distribution of carbon between these molecules may subsequently influence the production efficiencies of more complex organic molecules in regions of active star formation, where the ices are subject to heating, irradiation and shocks. By studying a range of absorbers, from pristine ices in dark clouds to processed ices in the vicinity of embedded stars, we will build a clear picture of the evolution of ices from the interstellar medium to protostellar envelopes and protoplanetary disks.