Astron. Astrophys. 332, 1099-1122 (1998)

Chemical reactions in protoplanetary accretion disks

IV. Multicomponent dust mixture

H.-P. Gail

Institut für Theoretische Astrophysik, Universität Heidelberg, Tiergartenstraße 15, D-69121 Heidelberg, Germany (gail@ita.uni-heidelberg.de)

Received 24 October 1997 / Accepted 2 January 1998

Abstract

We consider the different major components of the dust mixture in protostellar accretion disks and the development of their structure and chemical composition as the disk material slowly migrates inwards during the viscous phase of the disk evolution. It is shown that the amorphous structure of the dust grains from the parent molecular cloud is converted by annealing at about 800 K into a well ordered crystalline lattice structure accompanied by a chemical differentiation by solid state diffusion processes. The chemical composition of the abundant refractory dust components formed from silicon, magnesium, iron, aluminium, and calcium is discussed on the basis of chemical equilibrium considerations. Convenient approximations for calculating the equilibrium abundance of the major dust components are derived. These are used to calculate a self consistent model for a stationary accretion disk around a one solar mass protosun in the one zone approximation and to derive the radial variation of the abundance of the different dust species. The model takes properly into account the strong coupling between disk structure, opacity, and the chemical composition and abundance of the major dust species, i.e. amorphous silicates in the cool parts of the disk, and olivine, orthopyroxene, iron, and aluminium compounds in the warm parts.

Key words: accretion, accretion disks molecular processes solar system: formation dust, extinction

Send offprint requests to: H.-P. Gail

Contents

• 1. Introduction
• 2. Initial composition of the dust mixture
• 3. Annealing and diffusion I
  • 3.1. Silicate grains
  • 3.2. Iron grains
  • 3.3. Carbonaceous grains
• 4. Equilibrium chemistry of the Mg-Fe-complex
  • 4.1. Pure metals
  • 4.2. Siliconoxide
  • 4.3. Iron sulphide
  • 4.4. Iron oxides and hydrides
  • 4.5. Equilibrium chemistry of the Mg-Fe ortho-silicates
  • 4.6. The Mg-Fe meta-silicates
  • 4.7. Iron-magnesium-silicon compounds in the disk
• 5. Equilibrium chemistry of the Al-Ca complex
  • 5.1. Aluminium oxide
  • 5.2. Melilite
  • 5.3. Spinel
  • 5.4. Diopside
  • 5.5. Anorthite
  • 5.6. Aluminium compounds in the disk
• 6. Model calculation for a protoplanetary disk
  • 6.1. Equations for the disk structure
  • 6.2. Absorption by dust grains
• 7. Annealing and diffusion II
• 8. Disk model
• 9. Concluding remarks
• Acknowledgements
• References

© European Southern Observatory (ESO) 1998

Online publication: March 30, 1998
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