Infrared Space Observatory with the observations of solid carbon dioxide in molecular clouds

Spectra of interstellar CO2 ice absorption features at a resolving power of lambda/Delta lambda approximate to 1500-2000 are presented for 14 lines of sight. The observations were made with the Short-Wavelength Spectrometer ( SWS) of the Infrared Space Observatory (ISO). Spectral coverage includes th e primary stretching mode of CO2 near 4.27 mu m in all sources; the bending mode near 15.2 mu m is also detected in 12 of them. The selected sources i nclude massive protostars (Elias 29 [in rho Oph], GL 490, GL 2136, GL 2591, GL 4176, NGC 7538 IRS 1, NCC 7538 IRS 9, S140, W3 IRS 5, and W33 A), sourc es associated with the Galactic Center (Sgr A*, GCS 3 I, and GCS 4), and a background star behind a quiescent dark cloud in Taurus (Elias 16); they th us probe a diverse range of environments. Column densities of interstellar CO2 ice relative to H2O ice fall in the range 10%-23%: this ratio displays remarkably little variation for such a physically diverse sample. Compariso n of the observed profiles with laboratory data for CO2-bearing ice mixture s indicates that CO2 generally exists in at least two phases, one polar (H2 O dominant) and one nonpolar (CO2 dominant). The observed CO2 profiles may also be reproduced when the nonpolar components are replaced with thermally annealed ices. Formation and evolutionary scenarios for CO2 and implicatio ns for grain mantle chemistry are discussed. Our results support the conclu sion that thermal annealing, rather than energetic processing due to UV pho tons or cosmic rays, dominates the evolution of CO2-bearing ices.