2007ApJ...655..332W

We present new observations with the Infrared Spectrograph on board the Spitzer Space Telescope of the solid-CO₂absorption feature near 15 µm in the spectra of eight field stars behind the Taurus complex of dark clouds. Solid CO₂is detected in six lines of sight. New results are combined with previous data to investigate the correlation of CO₂column density with those of other major ice constituents (H₂O and CO) and with extinction. CO₂is shown to display a ``threshold extinction'' effect, i.e., a minimum extinction (A<sub>0</sub>=4.3±1.0 mag) required for detection, behavior similar to that previously reported for H<sub>2</sub>O and CO. We find a particularly tight correlation through the origin between N(CO<sub>2</sub>) and N(H<sub>2</sub>O), confirming that these species form in tandem and coexist in the same (polar) ice layer on the grains. The observed composition of the mantles is broadly consistent with the predictions of photochemical models with diffusive surface chemistry proposed by Ruffle & Herbst. Comparison of our results for Taurus with published data for Serpens indicates significant differences in ice composition consistent with enhanced CO<sub>2</sub>production in the latter cloud. Our results also place constraints on the distribution of elemental oxygen between ices and other potential reservoirs. Assuming a constant N(H) to extinction ratio, we show that ∼65% of the solar O abundance is accounted for by summing the contributions of ices (∼26%), refractory dust (∼30%) and gas-phase CO (∼9%). If the Sun is an appropriate standard for the interstellar medium, the ``missing'' oxygen may reside in atomic O I gas and/or (undetected) O₂ within the ices.