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 m in the spectra of eight field stars behind the Taurus complex of dark clouds. Solid CO2isdetectedinsixlinesofsight.Newresultsarecombinedwithpreviousdatatoinvestigatethecorrelationof CO2 columndensitywiththoseofother majoriceconstituents(H2OandCO)andwithextinction.CO2isshowntodisplaya ''thresholdextinction'' effect, i.e., a minimumextinction(A0 ¼ 4:3 1:0 mag)requiredfordetection,behaviorsimilar 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 observedcomposition of themantlesisbroadlyconsistentwiththepredictionsof photochemical modelswithdiffusive surface chemistry proposed by Ruffle & Herbst. Comparison of our results for Taurus with published data for Serpens indicatessignificantdifferencesinicecomposition consistentwithenhancedCO2productioninthe lattercloud.Ourre- sultsalsoplaceconstraintsonthedistributionofelementaloxygenbetweenicesandotherpotentialreservoirs.Assuming aconstant N(H)toextinctionratio,weshowthat ~65%of thesolarOabundanceisaccountedforbysummingthecontri- butions 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.