Mineral formation in stellar winds

Abstract

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.

Mineral formation in stellar winds

“…In a stellar wind, however, the process of dust formation of the silicates does not run into completion until the gas has cooled below the stability limit of MgO and a significant fraction of the Mg is left over in the gas phase at this instant to form MgO. Our previous model calculations (Gail & Sedlmayr 1999;Ferrarotti & Gail 2001) have already shown that because of the nonequilibrium nature of condensation in stellar outflows a certain quantity of solid MgO should be formed. Since the MgO forms a solid solution with FeO it is more likely, however, that in the presence of Fe vapour in the gas phase magnesiowüstite Mg x Fe 1−x−δ is formed in the outflow and not pure MgO.…”

“…The consumption of H 2 O, Fe, and Mg from the gas phase by formation of magnesiowüstite has to be considered analogous to Eqs. (51), (53), (54) of Ferrarotti & Gail (2001).…”

“…The calculations are based on the simplified wind model already used in the first three papers of this series (Gail & Sedlmayr 1999;Ferrarotti & Gail 2001: A stationary, spherically symmetrically wind is flowing out of the stellar atmosphere from a star with…”

“…The dust components considered in our previous non-equilibrium condensation calculations were olivine (Mg 2x Fe 2(1−x) SiO 4 ), pyroxene (Mg x Fe 1−x SiO 3 ), iron (Fe), quartz (SiO 2 ), and periclase (MgO) (Gail & Sedlmayr 1999;Ferrarotti & Gail 2001. Details of how the nonequilibrium condensation of these dust components is calculated are described in the previous papers and are not repeated at this place.…”

Dust input from AGB stars in the Large Magellanic Cloud

“…Ferrarotti & Gail (2006) performed extensive calculations of dust formation by low-and intermediate-mass stars during their AGB evolution for a wide range of initial stellar masses and metallicities, making these models the most suitable for use in evolutionary dust models. Their work is based on detailed theoretical models of non-equilibrium dust condensation in stellar winds, developed in a series of earlier papers (Gail & Sedlmayr 1987Ferrarotti & Gail 2001Gail 2003). These studies consider the formation of a multicomponent dust mixture in stationary, dust-driven winds with O-, C-, and S-chemistries.…”