Astrophys. J., 678, 1005-1031 (2008/May-2)
The c2d Spitzer spectroscopic survey of ices around low-mass young stellar objects. II. CO2.
PONTOPPIDAN K.M., BOOGERT A.C.A., FRASER H.J., VAN DISHOECK E.F., BLAKE G.A., LAHUIS F., OBERG K.I., EVANS II N.J. and SALYK C.
Abstract (from CDS):
This paper presents Spitzer IRS λ/Δλ∼600 spectroscopy of the CO215.2 µm bending mode toward 50 embedded young low-mass stars, taken mostly from the ``Cores to Disks'' (c2d) Legacy program. The average abundance of solid CO2 relative to water in low-mass protostellar envelopes is 0.32±0.02, significantly higher than that found in quiescent molecular clouds and in massive star-forming regions. A decomposition of the observed CO2 bending mode profiles requires a minimum of five unique components. Roughly (2)/(3) of the CO2 ice is found in a water-rich environment, while most of the remaining (1)/(3) is found in a CO environment with strongly varying relative concentrations of CO2 to CO along each line of sight. Ground-based observations of solid CO toward a large subset of the c2d sample are used to further constrain the CO2:CO component and suggest a model in which low-density clouds form the CO2:H2 O component and higher density clouds form the CO2:CO ice during and after the freezeout of gas-phase CO. The abundance of the CO2:CO component is consistent with cosmic-ray processing of the CO-rich part of the ice mantles, although a more quiescent formation mechanism is not ruled out. It is suggested that the subsequent evolution of the CO2 and CO profiles toward low-mass protostars, in particular the splitting of the CO2 bending mode due to pure, crystalline CO2, is first caused by distillation of the CO2:CO component through evaporation of CO due to thermal processing to ∼20-30 K. The formation of pure CO2 via segregation from the H2 O rich mantle may contribute to the band splitting at higher levels of thermal processing (≳50 K) but is harder to reconcile with the physical structure of protostellar envelopes around low-luminosity objects.
Astrochemistry - Stars: Circumstellar Matter - ISM: Dust, Extinction - ISM: Evolution
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