Radial mixing in protoplanetary accretion disks

Abstract

Abstract.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 ≈ 800 K, of combustion of the carbon dust component at about T ≈ 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 α-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 Tc, viscosity ν, . . . ) 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.

Gas-Phase CO in Protoplanetary Disks: A Challenge for Turbulent Mixing

“…There are many observational facts indicative of both large-scale and small-scale radial mixing, e.g, the presence of crystalline dust in comets (Bockelée-Morvan et al 2002;Wooden et al 2005) and protoplanetary disks (Gail 2001;van Boekel et al 2004). Radial mixing has also been invoked to explain the distribution of water ice in the Solar Nebula (Cyr et al 1998), deuteration in the Solar Nebula (Hersant et al 2001), and isotopic abundances in meteorites (Boss 2004).…”

Radial mixing in protoplanetary accretion disks

“…In the first part of the present series (Gail 2001a, henceforth called Paper I) stationary numerical models of protoplanetary accretion disks have been calculated which consider the combustion of carbon dust, annealing of silicate dust and radial mixing of the dust species. As shown in Paper I, carbon combustion, annealing of silicates and radial mixing have considerable influence on the chemical composition of the disk matter as well as on the radial disk structure.…”

Interstellar dust within the life cycle of the interstellar medium

“…The dust in the ISM is amorphous and the presence of forsterite in proto-planetary disks and in comets implies that the interstellar dust have been processed to high temperature to crystallise it. This may happen if the dust gets closer to the central star where it is annealed or completely destroyed and then re-condensed [110,111]. However, some mechanisms must exist, such as radial mixing of the nebulae and disks, to transport the dust crystallised near the star toward the outer cold region where it is observed and where comets form [112].…”