Spitzer Observations of CO2 Ice Towards Field Stars in the Taurus   Molecular Cloud
  We present the first Spitzer Infrared Spectrograph observations of the 15.2 micron bending mode of CO2 ice towards field stars behind a quiescent dark cloud. CO2 ice is detected towards 2 field stars (Elias 16, Elias 3) and a single protostar (HL Tau) with anabundance of ~15-20% relative to water ice. CO2 ice is not detected towards the source with lowest extinction in our sample, Tamura 17 (A_V = 3.9m). A comparison of the Elias 16 spectrum with laboratory data demonstrates that the majority of CO2 ice is embedded in a polar H2O-rich ice component, with ~15% of CO2 residing in an apolar H2O-poor mantle. This is the first detection of apolar CO2 towards a field star. We find that the CO2 extinction threshold is A_V = 4m +/- 1m, comparable to the threshold for water ice, but significantly less than the threshold for CO ice, the likely precursor of CO2. Our results confirm CO2 ice forms in tandem with H2O ice along quiescent lines of sight. This argues for CO2 ice formation via a mechanism similar to that responsible for H2O ice formation, viz. simple catalytic reactions on grain surfaces. 
Edwin A. Bergin, Gary J. Melnick, Perry A. Gerakines, David A.   Neufeld, Douglas C.B. Whittet