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Astron. Astrophys. 351, 1066-1074 (1999)

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7. Conclusion

The study of the 15.2 µm CO2 bending mode profile in RAFGL7009S as well as in other objects reveals that, in the ice mantles, intermolecular interactions are responsible for the peculiar line shape observed. We have been able to constrain the interaction on the basis of experiments and show that it originates from the interaction between a Lewis base and acid. This study leads to the detection of the formation of interstellar intermolecular complexes between the carbon dioxide and methanol molecules present in roughly similar abundances in the grain mantles. The CO2 bending mode line shape traces this physical interaction in the ice mantle, but is not sufficient in itself to uniquely identify the interacting molecule. If we limit ourselves to the good agreement between the two spectra in Fig. 6, we may expect that ethanol is rather abundant in ice mantles. This is not the case, regarding the entire infrared spectrum. Additional constraints on the possible candidates (cosmic abundance, laboratory investigations and specific modes vibrations expected from the candidate in other infrared regions) are needed to claim for a definitive identification. Methanol is this candidate.

The CO2 profiles observed in various lines of sight show evidence for both complex formation and thermochemical evolution of the ice. A sensitive probe of this evolution is given in the laboratory by the 13CO2 isotope stretching mode line shape which indicates the degree of segregation in ices. The profile analysis in all the known sources and their comparison is another step to understand ice evolution around young stellar objects.

The inferred candidate interacting with CO2 (i.e. CH3OH) allows to explain the global spectrum of RAFGL7009S. Methanol is responsible for about 30 to 60% of the so-called "6.85 µm" band and partly contributes to one fourth to one third of the feature at 4.9 µm attributed to OCS.

It seems paradoxical that the "6.85 µm" band in this source, that we detected since the very first ISO observations, was not attributed to methanol. Infrared vibrational spectroscopy of unknown solids often offers a limited diagnostics because blending of lines coming from different candidates may lead to non-unique assessment of the spectra. This is a serious limitation in the analysis of the profiles of infrared bands using only very narrow wavelength regions. However, a careful interpretation of laboratory experiments can help to overcome this difficulty, as we have shown in this paper with the fine tuning interpretation of the complex and blended "6.85 µm" bands.

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© European Southern Observatory (ESO) 1999

Online publication: November 16, 1999
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