Radial mixing in protoplanetary accretion disks - I. Stationary disc models with annealing and carbon combustion

The interplay between radial mixing process in protoplanetary accretion discs with processes leading to destruction or modification of the extinction properties of abundant dust species has significant consequences for the properties of the disk. This paper studies the consequences of annealing amorphous silicate dust at T approximate to 800 K, of combustion of the carbon dust component at about T approximate to 1000 K and of mixing the products into cold outer disc regions out to 10 AU and beyond. A model calculation in the one-zone approximation for stationary Keplerian alpha -disks around a solar-like protostar is combined with a solution of the equations for annealing of silicate dust grains, for carbon dust oxidation, and a solution of the diffusion equations for radial mixing of the dust components in the disc by turbulent flows. It is shown that annealing of amorphous silicate dust reduces the mass extinction coefficient of the disc matter by more than an order of magnitude in the warm disc zone. Radial mixing of the freshly produced crystalline silicate dust into outer disc regions reduces the opacity of the disc material also in cold disc regions where annealing is not possible. Mixing of carbon dust free material from the zone of carbon combustion into outer disc regions also leads to a considerable reduction of the opacity of the disc material. Radial mixing processes then modify the dust composition of the outer disc regions and by means of the dependence of the disk properties (midplane temperature T-c, viscosity nu,...) on the opacity also modify the structure and evolution of a protoplanetary disc. It is shown that turbulent mixing processes in the protoplanetary accretion disc of a Solar System like system during its evolution prior to the onset of the formation of planetary bodies carry material from inner disc regions r<1 AU outwards to at least 10...20 AU. This offers a simple explanation of the findings that a significant fraction of the cometary silicate dust grains are crystallised and that the matrix material of primitive meteorites contains thermally processed crystalline dust material.