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Astron. Astrophys. 349, 243-252 (1999)

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6. Summary and conclusions

Carbon bearing grains are expected to form in the outflows of C-rich AGB stars. The two most common types of carbon grains in these stars are expected to be amorphous carbon and SiC grains. We have investigated the direct influence of different dust optical properties on the wind characteristics and the resulting observable properties of the dynamical models.

The term amorphous carbon covers a wide variety of material properties from "diamond-like" to "graphite-like". We have used n and k data of Maron (1990), Rouleau & Martin (1991), Preibisch et al. (1993), Zubko et al. (1996) and Jäger et al. (1998) to investigate the influence of different types of amorphous carbon, on the structure and the wind properties of dynamical models. The Rosseland and Planck mean values of [FORMULA] used in the model computations were calculated from the Rayleigh approximation for spheres. The difference between the Planck and the Rosseland means is relatively small for the amorphous carbon data because the grains have a continuous opacity with a smooth wavelength dependence.

In our test models, both the outflow velocity [FORMULA] and degree of condensation [FORMULA] change significantly with the dust data used. [FORMULA] increases with decreasing dust extinction efficiency while [FORMULA] decreases, reflecting a lower optical depth of the circumstellar dust shell. The mass loss rate is, however, not significantly influenced by the use of different dust data.

On top of the structures resulting from the dynamic calculations we have performed detailed radiative transfer calculations to obtain the spectral energy distribution of the circumstellar dust shells. Regarding infrared colours, the influence of the different dust data used in the radiative transfer calculation is much stronger than the effect of the underlying hydrodynamic model structure. However, this should not be used as an excuse for fitting observations by arbitrarily choosing the optical properties of the dust grains for a given model structure. In a consistent model the dynamical properties (e.g. outflow velocities) and the optical appearance of the circumstellar envelope are related in a complex way.

The influence of including SiC grains is that the 11.3 µm feature appears in the spectral energy distribution of the models. How much SiC should be "mixed" into a model to reproduce the 11.3 µm feature observed (e.g. class 4 in Goebel et al. 1995) will very much depend on the assumptions which are made about the size and shape of the SiC grains which enter into the model (Papoular et al. 1998; Andersen et al. 1999; Mutschke et al. 1999).

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© European Southern Observatory (ESO) 1999

Online publication: August 25, 1999