JWST Observations of Segregated ¹²CO₂ and ¹³CO₂ Ices in Protostellar Envelopes

The evolution of interstellar ices can be studied with thermal tracers such as the vibrational modes of CO₂ ice that show great diversity depending on their local chemical and thermal environment. Now with the wide spectral coverage and sensitivity of the James Webb Space Telescope we can obtain observations of the weak and strong CO₂ absorption features inhabiting the near- and mid-infrared spectral region. In this work we present observations of the 15.2 μm bending mode, the 4.39 μm stretching mode and the 2.70 μm combination mode of ¹²CO₂ and ¹³CO₂ ice in the high-mass protostar IRAS 20126 and the low-mass protostar Per-emb 35, two sources that show clear signs of ice heating. The 15.2 μm bending mode of both protostars shows the characteristic double peak profile that is associated with pure CO₂ ice and a sharp short-wavelength peak is observed at 4.38 μm in the ¹³CO₂ bands of the two sources. Furthermore, a narrow short-wavelength feature is detected at 2.69 μm in the ¹²CO₂ combination mode of Per-emb 35. We perform a consistent profile decomposition on all three vibrational modes and show that the profiles of all three bands can be reproduced with the same linear combination of CO₂ ice in mixtures with mostly CH₃OH and H₂O ices when the ices undergo segregation due to heating. The findings show that upon heating, CO₂ ice is likely segregating from mostly the water-rich ice layer and the CO₂-CH₃OH component becomes dominant in all three vibrational modes. Additionally, we find that the contribution of the different CO₂ components with respect to the total absorption band is similar for both ¹²CO₂ and ¹³CO₂. This indicates that fractionation processes must not play a significant role during the different formation epochs, H₂O-dominated and CO-dominated, of the CO₂ ices and that the ratio persists through the heating stage. We quantify the ¹²CO₂ and ¹³CO₂ ice column densities and derive ¹²C/¹³C _ice = 90 ± 9 in IRAS 20126, a value that is lower compared to what was previously reported for warm gaseous CO in this source. Finally, we report the detection of the ¹³CO₂ bending mode of pure CO₂ ice at 15.64 μm in both IRAS 20126 and Per-emb 35.