The ¹²CO₂ and ¹³CO₂ Absorption Bands as Tracers of the Thermal History of Interstellar Icy Grain Mantles

Analyses of infrared signatures of CO₂ in water-dominated ices in the ISM can give information on the physical state of CO₂ in icy grains and on the thermal history of the ices themselves. In many sources, CO₂ was found in the "pure" crystalline form, as signatured by the splitting in the bending mode absorption profile. To a large extent, pure CO₂ is likely to have formed from segregation of CO₂ from a CO₂:H₂O mixture during thermal processing. Previous laboratory studies quantified the temperature dependence of segregation, but no systematic measurement of the concentration dependence of segregation is available. In this study, we measured both the temperature dependence and concentration dependence of CO₂ segregation in CO₂:H₂O mixtures, and found that no pure crystalline CO₂ forms if the CO₂:H₂O ratio is less than 23%. Therefore, the segregation of CO₂ is not always a good thermal tracer of the ice mantle. We found that the position and width of the broad component of the asymmetric stretching vibrational mode of ¹³CO₂ change linearly with the temperature of CO₂:H₂O mixtures, but are insensitive to the concentration of CO₂. We recommend using this mode, which will be observable toward low-mass protostellar envelopes and dense clouds with the James Webb Space Telescope, to trace the thermal history of the ice mantle, especially when segregated CO₂ is unavailable. We used the laboratory measured ¹³CO₂ profile to analyze the ISO-SWS observations of ice mantles toward Young Stellar Objects, and the astrophysical implications are discussed.