The abundance of carbon dioxide ice in the quiescent intracloud medium

We present new observations with the Infrared Spectrograph on board the Spitzer Space Telescope of the solid-CO2 absorption feature near 15 mu m in the spectra of eight field stars behind the Taurus complex of dark clouds. Solid CO2 is detected in six lines of sight. New results are combined with previous data to investigate the correlation of CO2 column density with those of other major ice constituents ( H2O and CO) and with extinction. CO2 is shown to display a threshold extinction'' effect, i. e., a minimumextinction ( A(0)=4.3 +/- 1.0 mag) required for detection, behavior similar to that previously reported for H2O and CO. We find a particularly tight correlation through the origin between N( CO2) and N( H2O), 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 CO2 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 similar to 65% of the solar O abundance is accounted for by summing the contributions of ices (similar to 26%), refractory dust (similar to 30%) and gas- phase CO (similar to 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) O-2 within the ices.