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.
- ISM: molecules
- methods: laboratory: molecular
- methods: laboratory: solid state
ASJC Scopus subject areas
- Astronomy and Astrophysics
- Space and Planetary Science