DETECTION OF ABUNDANT CO2 ICE IN THE QUIESCENT DARK CLOUD MEDIUM TOWARD ELIAS-16

We report the first detection of solid carbon dioxide (CO2) in quiesce nt regions of a dark cloud in the solar neighborhood, a result that ha s important implications for models of ice formation and evolution in the interstellar medium. The K-type field star Elias 16 was previously known to display solid-state absorption features of H2O and CO ices a rising in the Taurus Dark Cloud. Our detection of the CO2 feature at 4 .27 mu m in this line of sight implies a column density N(CO2) = 4.6(- 0.6)(+1.3) x 10(17) cm(-2), equivalent to similar to 18% and 70% of th e H2O and CO column densities, respectively. Comparison with laborator y data indicates that (unlike CO) the CO2 resides primarily in a polar (H2O-rich) component of the ices. CO2 is formed easily in the laborat ory by the photolysis of ice mixtures containing CO2 but the detection toward Elias 16 indicates that CO2 formation can occur in dark clouds in the absence of a local embedded source of radiation. Possible alte rnative mechanisms for CO2 production include grain surface reactions and energetic processing driven by the interstellar radiation field or cosmic rays.