Spitzerinfrared Spectrograph Observations of Class I/Ii Objects in Taurus: Composition and Thermal History of the Circumstellar Ices

We present observations of Taurus-Auriga Class I/II protostars obtained with the Spitzer InfraRed Spectrograph. Detailed spectral fits to the 6 and 15 micron features are made, using publicly-available laboratory data, to constrain the molecular composition, abundances, and levels of thermal processing along the lines of sight. We provide an inventory of the molecular environments observed, which have an average composition dominated by water ice with ~12% CO₂ (abundance relative to H₂O), >~2-9% CH₃OH, <~14% NH₃, ~4% CH₄, ~2% H₂CO, ~0. 6% HCOOH, and ~0. 5% SO₂. We find CO₂/H₂O ratios nearly equivalent to those observed in cold clouds and lines of sight toward the galactic center. The unidentified 6. 8 micron profiles vary from source to source, and it is shown to be likely that even combinations of the most common candidates (NH₄+ and CH₃OH) are inadequate to explain the feature fully. We discuss correlations among SED spectral indices, abundance ratios, and thermally-processed ice fractions and their implications for CO₂ formation and evolution. Comparison of our spectral fits with cold molecular cloud sight-lines indicates abundant prestellar ice environments made even richer by the radiative effects of protostars. Our results add additional constraints and a finer level of detail to current full-scale models of protostellar and protoplanetary systems.