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Astron. Astrophys. 322, 924-932 (1997)

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6. Conclusions

Radiative transfer modelling of circumstellar envelopes, based on simple assumptions as spherical symmetry and steady state outflows, allows detailed fitting of infrared data for single sources and the determination of system parameters as the distance, luminosity and total amount of dust ejected by the central star. The correlation with radio observations (giving informations of the total mass loss) will thus provide estimates of the dust to gas mass ratio for each modelled source. The accuracy of the fitting procedure depends mainly on the available opacities, expecially in the mid-IR range where the dust features are present; a good description of the mid-IR spectra is in fact essential for the determination of the envelope optical depth and chemical composition.

The necessity of accurate spectral modelling is becoming more and more important taking into account the better spectroscopic capabilities of ground based mid-IR cameras and ISO data.

Our models confirm that the shape of carbonaceous dust features are reproduced assuming a mixture of amorphous carbon, described by AC1 Rouleau & Martin (1991) opacity, and (10%, or less) Pégourié (1988) SiC. Minor components (as PAH) might be present, but the quality of IRAS LRS do not allow their detection.

Models for O-rich envelopes associated to M and S stars suggest the possibility of different varieties of silicate dust, even if mixing between oxidic and carbonaceous dust should be considered, at least in the case of transition objects as the S-stars. If the LML90 scheme is valid, we can recognize in the Ossenkopf 1 "dirty silicates" (OS92) the correct opacity for subclass "Sil" silicates, associated to [FORMULA] LRS spectra and a mixture of Ossenkopf 1 "dirty silicates" (OS92) with a large amount of Rouleau & Martin (1991) AC1 amorphous carbon (in the ratio 1:4 for the subclass "S" sources). For the other subclasses accurate fits need opacities including different components of dust, other than silicates.

The dust-to-gas mass ratios we obtained are generally low (between 0.1 and 0.5 %), except in the case of the O-rich "Sil" source WX Ser (1.5%); there is a tendency for having lower dust-to-gas ratios for C-rich envelopes, but this result may be influenced by uncertaintes in the average grain radius (that can be different between oxidic and carbonaceous dust) and by the inaccurate fitting of the silicate features for O-rich sources other than "Sil" subclass. New models taking into account the global dynamics of the system and coupling the dusty component with the gaseous one will allow direct correlation between IR and radio data, and will increase the accuracy in the determination of the dust-to-gas ratio, giving better insight on the dust condensation processes responsible for the mass loss in AGB stars and the formation of circumstellar envelopes.

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© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998