A STUDY OF THE SIZE AND COMPOSITION OF DUST GRAINS IN THE CIRCUMSTELLAR ENVELOPE OF IRC+10216

We utilise a modelling technique in which a generalised two-stream Edd ington approximation is applied to the problem of the radiative transf er in a spherically symmetric dust shell. We calculate the emerging fl ux taking into account the effect of non-isotropic scattering, absorpt ion and thermal re-emission for a multiple grain size, multiple grain component distribution. We have applied our technique to the well know n carbon-rich star IRC +10 216; we have considered a model with grains of amorphous carbon and silicon carbide, and we have studied how diff erent grain size distributions affect the transport coefficients and t he calculated flux. In agreement with previous works, we reproduce qui te well the general spectral characteristics, spanning from lambda = 0 .55 mu m to 2 cm, assuming the dust envelope composed of small grains (with radius similar to 10(-2) mu m) of amorphous carbons. The presenc e of silicon carbide cannot be excluded, but it is not required to exp lain the feature at 11.4 mu m observed in the IRAS Low Resolution Spec trum: a one-component model including only amorphous carbons fits both the general characteristics and the feature at 11.4 mu m.