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TU Berlin

Mineral formation in stellar winds: I. Condensation sequence of silicate and iron grains in stationary oxygen rich outflows

H.-P. Gail, and E. Sedlmayr

Astronomy & Astrophysics, 347, 594-616 (1999)

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This paper considers the growth of circumstellar dust grains formed from the elements silicon, magnesium, and iron. The stability of olivine (Mg2xFe2(1-x)SiO4), quartz (SiO2), iron, and periclase (MgO) dust in a circumstellar environment is discussed. The role of exchange of Fe2+ and Mg2+ cations, solid diffusion of Fe2+ 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:

circumstellar matter -Stars: mass-loss - Stars: AGB and post-AGB - dust - Diffusion

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