Diploma Thesis
Staubbildung in den Hüllen um S-Sterne

Andrea Ferrarotti

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 newly synthesized 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. In this work we have investigated the condensation processes for the peculiar element mixture of 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-equlibrium dust condensation in the outflow is calculated for a simple wind model. The results of our calculation show that dust condensates in S-Stars.

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