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

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 <sup>2

1</sup> 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 (), quartz (SiO₂), iron, and periclase (MgO) dust in a circumstellar environment is discussed. The role of exchange of Fe and Mg cations, solid diffusion of Fe 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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Contents

• 1. Introduction
• 2. Equilibrium chemistry of the Si-Mg-Fe complex
  • 2.1. Stability of magnesium-iron silicates
    • 2.1.1. Free evaporation
    • 2.1.2. Chemisputtering
  • 2.2. Condensation of silicon oxides
  • 2.3. Condensation of periclase
  • 2.4. Condensation of iron
• 3. Grain growth
  • 3.1. Formation of condensation centres
  • 3.2. Dust species
  • 3.3. Growth of silicate dust grains
  • 3.4. Change of composition of olivine
  • 3.5. Composition of olivine at the onset of grain growth
  • 3.6. Growth of iron grains
  • 3.7. Growth of quartz grains
• 4. Solid diffusion and annealing
  • 4.1. Diffusion within dust grains
  • 4.2. Boundary condition at the surface
  • 4.3. Diffusion coefficient
  • 4.4. Annealing
• 5. Model calculation
  • 5.1. Model of the stellar wind
  • 5.2. Dust extinction
  • 5.3. Chemical composition of the gas phase
  • 5.4. Equations for the dust growth
  • 5.5. Numerical results
    • 5.5.1. Wind model
    • 5.5.2. Grain mixture
    • 5.5.3. Composition of the olivine
    • 5.5.4. Annealing
• 6. Concluding remarks
• Acknowledgements
• Appendix A: solution of the diffusion equation
• References

© European Southern Observatory (ESO) 1999

Online publication: June 30, 1999
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