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

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5. Conclusion

The main results presented here show that the triple peak structure of the 15.2 µm bending mode of solid CO2, observed with ISO in the SWS spectra of RAFGL7009S, as well as in other protostars, can be satisfactorily duplicated by laboratory experiments involving heated mixtures of H2O, CH3OH and CO2 ices.

The pronounced triple peak structure is explained in terms of the evolution of molecular complexes between a polar molecule (such as CH3OH) and CO2 at relatively high temperature ([FORMULA] 50-60 K) in astronomical environments. All astronomical relevant molecules, possibly abundant in grain mantles, have been tested, some of them can give rise to this particular feature. From the abundance criteria, CH3OH is by far the best candidate. The excellent fit implies that probably all of the observed CO2 may be heated and involved in CH3OH-CO2 complexes, stabilized by H2O. It can not be excluded that some CO2, mixed with H2O ice, may be hidden in the bulk of the same band. If CH3OH is more abundant than CO2, as estimated for RAFGL7009S, the remaining amount of CH3OH may be present in pure form or mixed together with H2O ice. Finally, we wish to emphasize the excellent quality of the proposed fit, which strongly reinforces the validity of the laboratory approach in the interpretation of ISO data. In this object, at 15 µm, in the bending mode of CO2 ice, line shifts and shapes can be entirely described by physico-chemical interactions.

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© European Southern Observatory (ESO) 1998

Online publication: September 30, 1998