2008ApJ...678.1005P

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.