SWS observations of protostars: the relevance of laboratory ices simulations

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Interstellar Ices, Infrared Spectroscopy

Scientific paper

With its large wavelength coverage and a spectral resolution adequate for observations of solid state features, ISO is a unique and powerful tool. It allows not only to identify quantitatively simple and abundant molecular species frozen on grain surfaces but also to study their physico-chemical properties and their possible photochemical evolution towards molecular complexity. However, due to numerous and complicated solid state effects, the correct interpretation of the data requires the help of dedicated laboratory experiments, reproducing in a controlled environment ``interstellar ice mixtures''. These experiments, developed over the last 20 years, have allowed the detection of numerous icy solid state features even before ISO. Here we present the originality of ISO spectroscopic data. The continuous wavelength coverage, especially in the 2.5 - 25 μm range, the molecular vibrational region of the spectrum, coupled to comparisons with laboratory data, gives a deep insight into the solid state molecular species and their gas phase counterparts, in particular toward a carefully selected protostellar object, RAFGL 7009S. To illustrate our topics, we concentrate on this object which presents very deep molecular absorption features. H2O, CO, CO2 and CH4 are unambiguously identified. Other features are detected which are explained with further laboratory simulations involving photochemical and thermal processing of the ices as well as possible physical segregation of different ices made of large amounts of H2O, CO2 and CH3OH. As evidenced by the proposed fits, the relevance of the laboratory approach to the interpretation of ISO-SWS data is fully demonstrated. Finally, the large amounts of molecules detected in the solid phase around massive protostars raise the problem of the intrinsic chemical evolution of interstellar matter within this cold solid phase as opposed to the evolution in gas phase, the only one generally considered in the scheme of the chemical evolution of our Galaxy.

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