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Astron. Astrophys. 317, 859-870 (1997)

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6. Conclusions

One of the main aims of this study was to investigate what kind of dust is necessary to explain the unusual infrared spectrum observed from IRAS 22272 [FORMULA] 5435. We have found that PAHs (or molecules with very similar properties, e.g. hydrogenated solid amorphous carbon - see Buss et al. 1990) are essential for an interpretation of the spectral energy distribution. We also find that quantum treatment of their heating and cooling processes to be very important even for such a low temperature as 5300 K for the central star. It has been shown also that if the temperature of pure MgS-FeS grains is high enough we are able, in principle, to explain the observed 30 µm band by their IR emission, especially if we have a variety of shapes of grains inside the envelope. Until measurements of the absorption properties of MgS in the UV wavelengths become available, it will be rather difficult to rule out MgS as a possible carrier of the feature. On the other hand, sulfide mantles with amorphous carbon cores are found to be a less probable carrier of the feature as they possess two separate maxima with minimum around 30 µm in their absorption properties. The minimum disappears for thick coats, but this needs more sulphur than is available if coats are distributed proportionally to the surfaces of all AC grains present. Because the emitter(s) responsible for the 21 and 30 µm features are presently unknown, we have constructed an empirical opacity function which gave us a reasonable fit to the energy distribution between 18 and 48 µm.

The best fit constructed for IRAS 22272 allowed us to draw some conclusions concerning the evolutionary status of the source. We have shown that the estimated distance to this source is about 1.45 kpc if the mass of the central star is close to 0.60 [FORMULA]. The derived dynamical age of the nebula is then in agreement with the expected post-AGB evolution of such a hydrogen-burning star.

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© European Southern Observatory (ESO) 1997

Online publication: July 8, 1998