Dust driven winds in late supergiants

Abstract

The cool extended shells of giants and supergiants are well known to be places of copious dust formation as indicated by the occurrence of pronounced extinction, reddening, and polarization of the continuous star light and by the appearance of particular absorption features, both manifesting the interaction of photons with particles considerably larger than atoms or molecules. The accepted explanation of these phenomena is the formation of circumstellar dust, i.e. small solid particles with a typical size of the order of 0.1 micron. However, the analysis of these effects yields only information on the interaction of photons with certain functional groups within the clusters e.g. Si-O, C-H, C-C bending and Si-O, C-C, ... stretching vibrations and thus allows no definite determination of the "real" physical structure and chemical composition of the grain particles. Therefore, observational conclusions concerning the properties of circumstellar dust can provide only some "mean" information which allows no definitive conclusions regarding the true nature of the observed grains (geometrical shape, crystalline structure, ...?). An adequate answer to these questions must be based on the detailed study of the processes of formation and growth of dust particles in these environments. However, dust formation cannot be considered as an isolated problem because due to their huge absorption cross sections even a small contamination of the atmospheres by circumstellar dust may have a significant influence on the radiative transfer and (via energyand momentum-coupling) on the thermodynamic and hydrodynamic structure of the dust forming shell. Therefore, a consistent modelling of such objects requires the treatment of the coupled nonlinear system comprising radiative transfer density, velocity, temperature structure of the shell chemistry and dust formation. This has been pointed out by several authors in the past (e.g. Wickramasinghe, 1972; Kwok, 1975; Salpeter, 1974; Deguchi, 1980; Jura, 1983), but