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Astron. Astrophys. 328, 649-669 (1997)

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Infrared spectroscopy of interstellar apolar ice analogs

P. Ehrenfreund 1, A.C.A. Boogert 2, P.A. Gerakines 1, 3, A.G.G.M. Tielens 4 and E.F. van Dishoeck 1

1 Leiden Observatory, P.O. Box 9513, 2300 RA Leiden, The Netherlands
2 Kapteyn Astronomical Institute, P.O. Box 800, 9700 AV Groningen, The Netherlands
3 Department of Physics, Applied Physics & Astronomy, Rensselaer Polytechnic Institute, Troy, NY 12180-3590, USA
4 NASA Ames Research Center, Mail Stop 245-6, Moffett Field, CA 94035, USA

Received 23 April 1997 / Accepted 22 July 1997


Apolar ices have been observed in several regions in dense clouds and are likely dominated by molecules such as CO, CO2 and the infrared inactive molecules O2 and N2. Interstellar solid CO has been well characterized by ground-based high resolution measurements. Recent ISO results showed the ubiquitous presence of abundant CO2 ice and the presence of CO2 -rich ice mantles towards several molecular clouds. CO and CO2 have sharp bands in the infrared and their band shape depends strongly on the ice composition. The profiles of the strong CO and CO2 bands can therefore provide important information on the composition, temperature and thermal history of interstellar and precometary ices.

We address, in this paper, the infrared spectra of 70 apolar ice mixtures of pure, binary, and multicomponent type. We studied their spectral properties at 10 K, during warm up and UV photolysis, and derived the optical constants. We discuss the importance of particle shape calculations for strong transitions such as CO and CO2.

In the laboratory context, we investigate the formation of CO2 in the interstellar medium by UV photolysis of interstellar ices. Together with astronomical spectra taken by the ISO satellite these laboratory data will be extremely valuable for the determination of the grain mantle composition in dense clouds.

Key words: ISM: molecules; dust – infrared: interstellar: lines – methods: laboratory

Send offprint requests to: P. Ehrenfreund

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© European Southern Observatory (ESO) 1997

Online publication: March 26, 1998