Spitzer observations of CO2 ice toward field stars in the taurus molecular cloud

We present the first Spitzer Infrared Spectrograph observations of the 15.2 mu m bending mode of CO2 ice toward field stars behind a quiescent dark cloud. CO 2 ice is detected toward two field stars ( Elias 16 and Elias 3) and a single protostar (HL Tau) with an abundance of similar to 15% -20% relative to water ice. CO2 ice is not detected toward the source with lowest extinction in our sample, Tamura 17 ( A(v) = 3.9 mag). 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 similar to 15% of CO2 residing in an apolar, H2O-poor mantle. This is the first detection of apolar CO2 toward a field star. We find that the CO2 extinction threshold is Av = 4 +/- 1 mag, comparable to the threshold for water ice, but significantly less than the threshold for CO ice, the likely precursor of CO2. Our results confirm that CO2 ice forms in tandem with H2O ice along quiescent lines of sight. This argues for CO2 ice formation by means of a mechanism similar to that responsible for H2O ice formation, viz., simple catalytic reactions on grain surfaces.