ISO-SWS observations of interstellar solid (CO2)-C-13:heated ice and the Galactic C-12/C-13 abundance ratio

We present observations of the stretching mode of (CO2)-C-13 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 (CO2)-C-13 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 2282 cm(-1) (4.382 mu m), 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 C-12/C-13 ratio from the solid state as a function of Galacto-centric radius. The (CO2)-C-12/(CO2)-C-13 ratio for the local ISM (69+/-15), as well as the dependence on Galacto-centric radius, are in good agreement with gas phase ((CO)-O-18, H2CO) studies. For the few individual objects for which gas phase values are available, the C-12/C-13 ratios derived from CO2 tend to be higher compared to CO studies (albeit with similar to 2.5 sigma significance only). We discuss the implications of this possible difference for the chemical origin of interstellar CO2.