THE THERMAL EVOLUTION OF ICES IN THE ENVIRONMENTS OF NEWLY FORMED STARS: THE CO2 DIAGNOSTIC

Archival data from the Infrared Spectrometer of the Spitzer Space Telescope are used to study the 15 mu m absorption feature of solid CO2 toward 28 young stellar objects (YSOs) of approximately solar mass. Fits to the absorption profile using laboratory spectra enable categorization according to the degree of thermal processing of the ice matrix that contains the CO2. The majority of YSOs in our sample (20 out of 28) are found to be consistent with a combination of polar (H2O-rich) and nonpolar (CO-rich) ices at low temperature; the remainder exhibit profile structure consistent with partial crystallization as the result of significant heating. Ice-phase column densities of CO2 are determined and compared with those of other species. Lines of sight with crystallization signatures in their spectra are found to be systematically deficient in solid-phase CO, as expected if CO is being sublimated in regions where the ices are heated to crystallization temperatures. Significant variation is found in the CO2 abundance with respect to both H2O (the dominant ice constituent) and total dust column (quantified by the extinction, AV). YSOs in our sample display typically higher CO2 concentrations (independent of evidence for thermal processing) in comparison to quiescent regions of the prototypical cold molecular cloud. This suggests that enhanced CO2 production is driven by photochemical reactions in proximity to some YSOs, and that photoprocessing and thermal processing may occur independently.