CHEMICAL-REACTIONS IN PROTOPLANETARY ACCRETION DISKS - IV - MULTICOMPONENT DUST MIXTURE

We consider the different major components of the dust mixture in prot ostellar accretion disks and the development of their structure and ch emical composition as the disk material slowly migrates inwards during the viscous phase of the disk evolution. It is shown that the amorpho us structure of the dust grains from the parent molecular cloud is con verted by annealing at about 800 K into a well ordered crystalline lat tice structure accompanied by a chemical differentiation by solid stat e diffusion processes. The chemical composition of the abundant refrac tory dust components formed from silicon, magnesium, iron, aluminium, and calcium is discussed on the basis of chemical equilibrium consider ations. Convenient approximations for calculating the equilibrium abun dance of the major dust components are derived. These are used to calc ulate a self consistent model for a stationary accretion disk around a one solar mass protosun in the one zone approximation and to derive t he radial variation of the abundance of the different dust species. Th e model takes properly into account the strong coupling between disk s tructure, opacity, and the chemical composition and abundance of the m ajor dust species, i.e. amorphous silicates in the cool parts of the d isk, and olivine, orthopyroxene, iron, and aluminium compounds in the warm parts.