SIMBAD references

We present observations of the stretching mode of ¹³CO₂ ice along 13 lines of sight in the Galaxy, using the Short Wavelength Spectrometer on board of the Infrared Space Observatory (ISO-SWS). Remarkable variations are seen in the absorption band profile in the different lines of sight. The main feature is attributed to ¹³CO₂ mixed with polar molecules such as H₂O, and CH₃OH. The high-mass protostars GL 2136, GL 2591, S 140 : IRS1, and W 3 : IRS5 show an additional narrow substructure at 2282cm^–1 (4.382µm), which we attribute to a polar, CH₃OH-containing CO₂ ice, that experienced heating. This heating effect is sustained by a good correlation of the strength of the substructure with dust and CO gas temperatures along the line of sight, and anti-correlation with ice abundances. Thus, our main conclusion is that interstellar CO₂ ices around luminous protostars are subjected to, and altered by, thermal processing and that it may reflect the evolutionary stage of the nearby protostar. In contrast, the ices around low mass protostars and in a quiescent cloud in our sample do not show signs of thermal processing. Furthermore, we determine for the first time the Galactic ¹²C/¹³C ratio from the solid state as a function of Galacto-centric radius. The ¹²CO₂/¹³CO₂ ratio for the local ISM (69±15), as well as the dependence on Galacto-centric radius, are in good agreement with gas phase (C¹⁸O, H₂CO) studies. For the few individual objects for which gas phase values are available, the ¹²C/¹³C ratios derived from CO₂ tend to be higher compared to CO studies (albeit with ∼2.5 σ significance only). We discuss the implications of this possible difference for the chemical origin of interstellar CO₂.