1999A&A...350..240E

Data of the Infrared Space Observatory ISO have strongly influenced the current view of interstellar ice chemistry. ISO and ground-based results have confirmed that the most abundant ice species in warm regions close to massive protostars are H₂O, CO₂, and CH₃OH. Ice segregation in those environments reflects the extensive thermal processing of grains over the lifetime of protostars. We present here a systematic set of laboratory infrared spectra of ice mixtures dominated by H₂O, CO₂ and CH₃OH which have been exposed to thermal and UV irradiation processing. It is shown that the infrared bands of CO₂ and of CH₃OH are particularly sensitive to the ice composition, temperature and applied UV irradiation. The laboratory data suggest partial crystallization of interstellar ices in the protostellar environment. We present a detailed laboratory study of the CO₂ bending mode at 15.2µm. The observed multipeak structure of the CO₂ bending mode is a result of thermal processing and can not be produced by UV irradiation in the laboratory. Laboratory results show that annealed CO₂ ice has a lower stability against UV irradiation than cold amorphous CO₂ ice. Annealed ice mixtures containing H₂O, CO₂ and CH₃OH show that the multipeak structure of the CO₂ bending mode is not destroyed by UV fluxes of ≲10¹⁸photons/cm². Detailed analysis of H₂O, CO₂ and CH₃OH bands show that their profiles can be effectively used to trace the line of sight conditions and the origin and evolution of the ice composition in dense clouds. The datafiles discussed in this paper can be retrieved from the Leiden observatory database (http://www.strw.LeidenUniv.nl/~lab/isodb).