EDP Sciences
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Volume 367, Number 1, February III 2001
Page(s) 189 - 198
Section Formation, structure and evolution of stars
DOI http://dx.doi.org/10.1051/0004-6361:20000411

A&A 367, 189-198 (2001)
DOI: 10.1051/0004-6361:20000411

Modeling of C stars with core/mantle grains: Amorphous carbon + SiC

S. Lorenz-Martins1, F. X. de Araújo2, S. J. Codina Landaberry2, W. G. de Almeida2 and R. V. de Nader1

1  Observatorio do Valongo/UFRJ, Ladeira Pedro Antonio 43, 20080-090, Rio de Janeiro, Brazil
2  Observatorio Nacional/MCT, Rua Gal. Jose Cristino 77, 20921-400, Rio de Janeiro, Brazil

(Received 4 August 2000 / Accepted 9 November 2000)

A set of 45 dust envelopes of carbon stars has been modeled. Among them, 34 were selected according to their dust envelope class (as suggested by Sloan et al. 1998) and 11 are extreme carbon stars. The models were performed using a code that describes the radiative transfer in dust envelopes considering core/mantle grains composed by an $\alpha$-SiC core and an amorphous carbon (A.C.) mantle. In addition, we have also computed models with a code that considers two kinds of grains -$\alpha$-SiC and A.C. - simultaneously. Core-mantle grains seem to fit dust envelopes of evolved carbon stars, while two homogeneous grains are more able to reproduce thinner dust envelopes. Our results suggest that there exists an evolution of dust grains in the carbon star sequence. In the beginning of the sequence, grains are mainly composed of SiC and amorphous carbon; with dust envelope evolution, carbon grains are coated in SiC. This phenomena could perhaps explain the small quantity of SiC grains observed in the interstellar medium. However, in this work we consider only $\alpha$-SiC grains, and the inclusion of $\beta$-SiC grains can perhaps change some of these results.

Key words: stars: carbon -- circumstellar matter -- radiative transfer

Offprint request: S. Lorenz-Martins,lorenz@sun1.ov.ufrj.br

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