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Astron. Astrophys. 350, 240-253 (1999)


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Laboratory studies of thermally processed H2O-CH3OH-CO2 ice mixtures and their astrophysical implications

P. Ehrenfreund 1,2, O. Kerkhof 1,2, W.A. Schutte 2, A.C.A. Boogert 5, P.A. Gerakines 3,4, E. Dartois 5, L. d'Hendecourt 5, A.G.G.M. Tielens 6, E.F. van Dishoeck 1 and D.C.B. Whittet 3

1 Leiden Observatory, P.O. Box 9513, 2300 RA Leiden, The Netherlands
2 Raymond and Beverly Sackler Laboratory for Astrophysics at Leiden Observatory, P.O. Box 9513, 2300 RA Leiden, The Netherlands
3 Department of Physics, Applied Physics and Astronomy, Rensselaer Polytechnic Institute, Troy, NY 12180-3590, USA
4 NASA/Goddard Space Flight Center, Mail Code 691, Greenbelt, MD 20771, USA
5 Institute d'Astrophysique Spatiale, Campus d'Orsay, F-91405 Orsay, France
6 Kapteyn Astronomical Institute, P.O. Box 800, 9700 AV Groningen, The Netherlands

Received 10 May 1999 / Accepted 19 July 1999

Abstract

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 15.2 µm. 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 [FORMULA] 1018 photons[FORMULA]cm-2. 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 (www.strw.leidenuniv.nl/[FORMULA]lab/isodb).

Key words: methods: laboratory – ISM: abundances – ISM: dust, extinction – ISM: evolution – ISM: molecules – infrared: ISM: lines and bands

Send offprint requests to: P. Ehrenfreund

© European Southern Observatory (ESO) 1999

Online publication: September 24, 1999

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