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

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1. Introduction

Asymptotic giant branch (AGB) stars show large amplitude pulsations with periods of about 100 to 1000 days. The pulsation creates strong shock waves in the stellar atmosphere, causing a levitation of the outer layers. This cool and relatively dense environment provides favourable conditions for the formation of molecules and dust grains. Dust grains play an important role for the heavy mass loss, which influences the further evolution of the star.

Condensation and evaporation of dust in envelopes of pulsating stars must be treated as a time-dependent process since the time scales for condensation and evaporation are comparable to variations of the thermodynamic conditions in the stellar envelope. The radiation pressure on newly formed dust grains can enhance or even create shock waves leading to more or less pronounced discrete dust shells in the expanding circumstellar flow (e.g. Fleischer et al. 1991, 1992; Höfner et al. 1995; Höfner & Dorfi 1997). Since a significant part of the dust grains transferred to interstellar space are produced in the atmosphere of these old luminous stars (Sedlmayr 1994) an understanding of the nature of mass loss of these long-period variables is crucial for the general understanding of dust in space.

Modelling circumstellar envelopes requires knowledge of the absorption properties of the different types of grains over the relevant part of the electromagnetic spectrum. For this the optical properties of the corresponding dust material are needed. Amorphous carbon is a very good candidate as the most common type of carbon grains present in circumstellar envelopes, since the far-infrared data of late-type stars show a spectral index as expected for a very disordered two-dimensional material like amorphous carbon (Huffman 1988).

Silicon carbide (SiC) grains seem to be another important component of the dust in circumstellar envelopes. While amorphous carbon could explain the continuum emission, SiC particles could be responsible for the 11.3 µm band observed in many C-rich objects.

In this paper, we present self-consistent dynamical models of circumstellar dust shells calculated with selected laboratory amorphous carbon data. Based on these models we have performed radiative transfer calculations for pure amorphous carbon and in some cases also including SiC dust. In Sect. 3 the used amorphous carbon data are described. The influence on the model structure is described in Sect. 4 and the resulting spectral appearance is discussed in Sect. 5.

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© European Southern Observatory (ESO) 1999

Online publication: August 25, 1999