The composition of the dust mixture and the amounts of dust returned into the interstellar medium during the evolution of low and intermediate mass stars on the AGB is calculated by simple models for dust-forming stellar winds with M-, S-, and C-star chemistry. These models consider the most abundant dust species formed in circumstellar dust shells, i.e. olivine-and pyroxene-type silicate grains and iron grains in the case of M-stars, iron grains in the case of S-stars, and carbon, SiC, and iron grains in the case of C-stars. The wind models are combined with calculations of synthetic AGB-evolution to consistently determine the variation of the dust injection rate into the interstellar medium from AGB-stars with initial mass and metallicity as the stars evolve along the AGB and change their chemical surface compositions due to repeated "dredge-up" episodes. Numerical results are presented for initial stellar masses from 1 to 7 M and for metallicities between Z = 0.001 and Z = 0.04, which can then be used to calculate chemical evolution models for the dust content of galaxies.

Abstract. S stars are transition objects at the top of the AGB between M-and C-stars. The third dredge-up in thermally pulsating AGB-stars transports freshly synthesised carbon from the He burning zone into the stellar atmosphere. If the carbon and oxygen abundance are about equal the star is classified as being of spectral type S. Stars at the top of the AGB are subject to large mass-loss caused by a strong stellar wind. At some distance from the star the temperature of the gas drops below the condensation temperature of some mineral compounds and soot. This results in optically thick dust shells which are sources of an intense IR-radiation from warm dust. The composition of the dust formed in the stellar outflow critically depends on the C/O-ratio. For the standard element mixture characteristic for main sequence stars one has C/O < 1 (M-stars) and the O not bound in CO forms a mineral assemblage which is dominated by Mg-Fe-silicates and metallic Fe. For C/O > 1 (C-stars) the dust mixture is dominated by solid carbon and some SiC. The element mixture of S stars is characterised by the non-availability of sufficient quantities of O or C to form the standard condensates. We have investigated the condensation processes for the peculiar element mixture at the M-S-C transition on the AGB. From thermodynamic equilibrium calculations we find that the most likely solids to be formed are solid FeSi, metallic iron and small quantities of forsterite and SiC. Nucleation of dust may be triggered by TiC, ZrC or TiO2. For these substances, non-equilibrium dust condensation in the outflow is calculated for a simple stationary wind model for a sample of C/O-ratios. The results of our calculation show that iron and FeSi dust condensates in the circumstellar shells of S stars.

Abstract. S stars are transition objects at the top of the AGB between M-and C-stars. The third dredge-up in thermally pulsating AGB-stars transports freshly synthesised carbon from the He burning zone into the stellar atmosphere. If the carbon and oxygen abundance are about equal the star is classified as being of spectral type S. Stars at the top of the AGB are subject to large mass-loss caused by a strong stellar wind. At some distance from the star the temperature of the gas drops below the condensation temperature of some mineral compounds and soot. This results in optically thick dust shells which are sources of an intense IR-radiation from warm dust. The composition of the dust formed in the stellar outflow critically depends on the C/O-ratio. For the standard element mixture characteristic for main sequence stars one has C/O < 1 (M-stars) and the O not bound in CO forms a mineral assemblage which is dominated by Mg-Fe-silicates and metallic Fe. For C/O > 1 (C-stars) the dust mixture is dominated by solid carbon and some SiC. The element mixture of S stars is characterised by the non-availability of sufficient quantities of O or C to form the standard condensates. We have investigated the condensation processes for the peculiar element mixture at the M-S-C transition on the AGB. From thermodynamic equilibrium calculations we find that the most likely solids to be formed are solid FeSi, metallic iron and small quantities of forsterite and SiC. Nucleation of dust may be triggered by TiC, ZrC or TiO2. For these substances, non-equilibrium dust condensation in the outflow is calculated for a simple stationary wind model for a sample of C/O-ratios. The results of our calculation show that iron and FeSi dust condensates in the circumstellar shells of S stars.

Abstract. An emission band around 92 µm found in a few IR spectra from highly evolved stars was proposed to be due to the presence of carbonate dust grains in the circumstellar material (Kemper et al. 2002a, Nature, 415, 295). This contribution presents the results of a model calculation for the condensation of calcite (CaCO 3 ) in the stellar wind of AGB stars. It is shown that the quantities of carbonate dust formed relative to the quantities of silicate dust are negligibly small. This results from the fact that carbonates form at a much lower temperature than the silicate dust components. Carbonate dust formation then is suppressed by the strong acceleration of the wind material by radiation pressure on the silicate dust and the subsequent rapid dilution of the wind material. This makes it highly improbable that carbonate dust can be formed in stellar outflows.

Abstract. The possible formation of iron-magnesium-oxides in stellar outflows from cool stars with oxygen rich element mixture is discussed. Chemical equilibrium calculations for the non-ideal solid solution magnesiowüstite with composition Mg x Fe 1−x−δ O show that this oxide may be formed under conditions of imperfect equilibrium condensation. A model is developed for calculating the condensation of magnesiowüstite under non equilibrium conditions in stellar outflows and numerical models for multi-component dust condensation including magnesiowüstite in oxygen rich stellar outflows from cool stars are calculated. It is found that magnesiowüstite should be formed in small but probably detectable quantities in outflows from oxygen rich AGB-stars with mass-loss ratesṀ < ∼ 4 × 10 −6 M yr −1 . It forms a feature in the spectrum which seems to correspond to a not yet identified feature seen in some circumstellar dust shells. For higher mass-loss rates condensation of magnesiowüstite is suppressed by early massive condensation of silicates.