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Astron. Astrophys. 347, 594-616 (1999)

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Mineral formation in stellar winds

I. Condensation sequence of silicate and iron grains in stationary oxygen rich outflows

H.-P. Gail 1 and E. Sedlmayr 2

1 Institut für Theoretische Astrophysik, Universität Heidelberg, Tiergartenstrasse 15, D-69121 Heidelberg, Germany (gail@ita.uni-heidelberg.de)
2 Institut für Astronomie und Astrophysik, Technische Universität Berlin, Hardenbergstrasse 36, D-10623 Berlin, Germany (sedlmayr@physik.tu-berlin.de)

Received 10 August 1998 / Accepted 23 March 1999

Abstract

This paper considers the growth of circumstellar dust grains formed from the elements silicon, magnesium, and iron. The stability of olivine ([FORMULA]), quartz (SiO2), iron, and periclase (MgO) dust in a circumstellar environment is discussed. The role of exchange of Fe[FORMULA] and Mg[FORMULA] cations, solid diffusion of Fe[FORMULA] cations within the SiO4 matrix of the silicate lattice, and annealing of an initially amorphous lattice structure during olivine growth is considered. The complete set of equations describing the vapourisation and growth of a mixture of olivine, quartz, iron, and periclase grains, including the internal diffusion and the surface exchange processes, is derived. These equations are solved for a simplified model of a stellar wind for the case of an M star. The calculation shows that for M stars the dust in the circumstellar shell is a multicomponent mixture dominated by olivine and iron grains. Olivine grains likely show variations of their magnesium and iron content between the core and the surface. Some periclase and a tiny fraction of quartz also are formed in the outflow.

Key words: stars: circumstellar matter – stars: mass-loss – stars: AGB and post-AGB

Send offprint requests to: H.-P. Gail

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© European Southern Observatory (ESO) 1999

Online publication: June 30, 1999
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