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Astron. Astrophys. 357, L13-L16 (2000)

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1. Introduction

Advanced stages of stellar evolution are generally accompanied by heavy mass-loss either by explosive events or during phases of a massive stellar wind. Tiny solid particles condense from the gas phase which are easely detected by their infrared emission. The chemical composition and the mineralogical properties of the condensed material are not easely determined since such dust cannot simply be analysed in the laboratory, except for the rare presolar grains detected in certain meteorites (see Zinner 1998 for a review). From the limited spectroscopic information it was, nontheless, possible to show that essentially two different compositions of solid material in circumstellar dust shells exist for stars loosing mass by a stellar wind: (i) stars with the standard cosmic element abundance form Mg-Fe-silicate dust and (ii) stars with a carbon rich element mixture form soot and SiC. In both cases the precise composition of the material remains an open question since the observable absorption or emission bands are rather unspecific. This situation changed since the ISO satellite came into operation, and now a lot of well resolved solid state bands are detected (e.g. Waters et al. 1996; Barlow 1998; Molster et al. 1999), especially in the formerly non-accessible IR region longwards of [FORMULA]m.

Calculations of condensation sequences for the relevant element mixtures have been used from the beginning to constrain the possible materials which are formed in the outflow. Gilman (1969) showed that olivine and pyroxene in M-stars and carbon dust and SiC in C-stars are expected to be the main dust components. More recent calculations for the oxygen rich mixture of M-stars (e.g. Grossman 1972; Sharp & Huebner 1990) and the carbon rich mixture of C-stars (e.g. Lodders & Fegley 1993; Bernatowicz et al. 1996) predict several additional dust materials to condense from the gas phase, some of which have now been detected spectroscopically or as presolar grains in meteorites.

In an attempt to determine the possible condensates of (i) S-stars during the transition from M- to C-stars and (ii) of LBV outflows, we calculated the chemical equilibrium compositions of gas-solid mixtures for the element compositions of such stars. We used the set of solids listed in the JANAF tables (Chase et al. 1985) and added additional compounds with high melting or boiling points from Barin (1992), Kubaschewski and Alcock (1983), especially FeSi, FeSi2, and Fe3Si7. We found that FeSi in chemical equilibrium is the first abundant condensate for the two peculiar element mixtures, which both are characterised by a lack of oxygen and of carbon available for mineral formation. FeSi, thus, is an unexpected new candidate for forming an abundant dust component in stars with the peculiar surface element compositions in highly evolved stars of medium and high mass stars.

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© European Southern Observatory (ESO) 2000

Online publication: May 3, 2000