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 l ines of sight in the Galaxy, using the Short Wavelength Spectrometer on boa rd of the Infrared Space Observatory (ISO-SWS). Remarkable variations are s een in the absorption band profile in the different lines of sight. The mai n feature is attributed to (CO2)-C-13 mixed with polar molecules such as H2 O, 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 experienc ed heating. This heating effect is sustained by a good correlation of the s trength of the substructure with dust and CO gas temperatures along the lin e of sight, and anti-correlation with ice abundances. Thus, our main conclu sion is that interstellar CO2 ices around luminous protostars are subjected to, and altered by, thermal processing and that it may reflect the evoluti onary 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 ther mal processing. Furthermore, we determine for the first time the Galactic C-12/C-13 ratio f rom 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, H 2CO) 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 compare d to CO studies (albeit with similar to 2.5 sigma significance only). We di scuss the implications of this possible difference for the chemical origin of interstellar CO2.