Detection of Abundant CO2 Ice in the Quiescent Dark Cloud Medium toward Elias 16
de Graauw, Th.; Prusti, T.; van Dishoeck, E. F.; Whittet, D. C. B.; Schutte, W. A.; Tielens, A. G. G. M.; Boogert, A. C. A.; Ehrenfreund, P.; Gerakines, P. A.; Vandenbussche, B.; Chiar, J. E.; Adamson, A. J.
United States, Netherlands, United Kingdom, Spain, Belgium
Abstract
We report the first detection of solid carbon dioxide (CO2) in quiescent regions of a dark cloud in the solar neighborhood, a result that has 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 arising in the Taurus Dark Cloud. Our detection of the CO2 feature at 4.27 μm in this line of sight implies a column density N(CO2)=4.6+1.3-0.6×1017 cm-2, equivalent to ~18% and 70% of the H2O and CO column densities, respectively. Comparison with laboratory data indicates that (unlike CO) the CO2 resides primarily in a polar (H2O-rich) component of the ices. CO2 is formed easily in the laboratory by the photolysis of ice mixtures containing CO, 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 alternative mechanisms for CO2 production include grain surface reactions and energetic processing driven by the interstellar radiation field or cosmic rays.
Based on observations with Infrared Space Observatory, a European Space Agency (ESA) project, with instruments funded by ESA Member States (especially the PI countries France, Germany, The Netherlands, and the UK) and with the participation of ISAS and NASA.