Brunken et al. 2025; 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 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 ratio persists through the heating stage. We
quantify the 12CO2 and 13CO2 ice column densities and derive 12C/13Cice = 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