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Astron. Astrophys. 339, L17-L20 (1998)

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1. Introduction

Molecular clouds are composed of interstellar gas and a small amount of interstellar dust. They contain different environments, such as dense cores, which are characterized by very low temperatures (10-30 K) and high densities ([FORMULA] hydrogen atoms cm-3). Such dense cores are the site of star formation and the regions where interstellar ices form. Astronomical observations indicate the existence of different types of ices in quiescent and protostellar environments and toward field stars (Tielens & Whittet 1997). Hydrogen-rich ices (polar ices), dominated by H2O ice, are formed when H is abundant in the interstellar gas. Hydrogen-poor ice (apolar ices), dominated by CO, are directly accreted from the gas phase in regions where CO is abundant, but H and O are depleted in the interstellar gas.

The ISO-SWS instrument offering a large wavelength coverage and a resolution well adapted to the solid phase is about to change our knowledge of the physical-chemical properties of ices in space. The discovery of many new ice features was reported and the comparison with dedicated laboratory experiments allowed to determine more accurate abundances of major ice components (Ehrenfreund et al. 1997a). ISO has confirmed the ubiquity of solid CO2 detected by IRAS-LRS in the spectra of 3 protostars (d'Hendecourt & Jourdain de Muizon 1989). A relative high abundance of solid CO2, namely 15-20% compared to H2O ice has been recently reported (de Graauw et al. 1996, d'Hendecourt et al. 1996, Guertler et al. 1996). The abundance of CH3OH has been a debated subject for several years (Allamandola et al. 1992, Skinner et al. 1992). Recent ground-based observations of CH3OH bands in the NIR (near-infrared) toward RAFGL7009S seem to confirm high methanol abundances toward some massive protostars (Dartois et al. 1998a). NH3 has recently been detected by ground-based observations with an estimated abundance of [FORMULA] 10% relative to water ice (Lacy et al. 1998). Traces of other species on the few% level for CH4, HCOOH and possibly H2CO have been reported from ISO spectra toward selected sources (see Ehrenfreund et al. 1997a and Schutte 1998 for a review).

In this letter, we present laboratory data that allow the exact reproduction of the CO2 bending mode observed toward RAFGL7009S. The results indicate that thermal processing of ices in the line of sight toward protostellar objects is an efficient process.

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© European Southern Observatory (ESO) 1998

Online publication: September 30, 1998