2005ApJ...627L..33B

We present the first Spitzer Infrared Spectrograph observations of the 15.2 µm bending mode of CO₂ice toward field stars behind a quiescent dark cloud. CO₂ice is detected toward two field stars (Elias 16 and Elias 3) and a single protostar (HL Tau) with an abundance of ∼15%-20% relative to water ice. CO₂ice 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 CO₂ice is embedded in a polar, H₂O-rich ice component, with ∼15% of CO₂residing in an apolar, H₂O-poor mantle. This is the first detection of apolar CO₂toward a field star. We find that the CO₂extinction threshold is A_V=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 CO₂ice forms in tandem with H₂O ice along quiescent lines of sight. This argues for CO₂ice formation by means of a mechanism similar to that responsible for H₂O ice formation, viz., simple catalytic reactions on grain surfaces.