The c2d Spitzer spectroscopic survey of ices around low-mass young stellar objects. II. CO₂

Pontoppidan, Klaus M.; Boogert, A. C. A.; Fraser, Helen J.; van Dishoeck, Ewine F.; Blake, Geoffrey A.; Lahuis, Fred; Öberg, Karin I.; Evans II, Neal J. and Salyk, Colette (2008). The c2d Spitzer spectroscopic survey of ices around low-mass young stellar objects. II. CO₂. Astrophysical Journal, 678(2) pp. 1005–1031.

DOI: https://doi.org/10.1086/533431

Abstract

This paper presents Spitzer IRS λ/∆λ ~ 600 spectroscopy of the CO₂ 15.2 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 CO₂bending mode profiles requires a minimum of five unique components. Roughly 2/3 of the CO₂ ice is found in a waterrich 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 :H₂O 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 CO₂bending 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 CO~ via segregation from the H₂O 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

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