How Dust Behaves in Protoplanetary Disks

Diversity in the outcome of dust radial drift in protoplanetary discs

Authors:

Pinte et al

Abstract:

The growth of dust particles into planet embryos needs to circumvent the radial-drift barrier, i.e. the accretion of dust particles onto the central star by radial migration. The outcome of he dust radial migration is governed by simple criteria between the dust-to-gas ratio and the exponents p and q of the surface density and temperature power-laws. The transfer of radiation provides an additional constraint between these quantities as the disc thermal structure is fixed by the dust spatial distribution. In order to assess which discs are preferentially affected by the radial-drift barrier, we use the radiative transfer code MCFOST to compute the temperature structure of a large range of disc models, stressing the particular effects of grain size distributions and vertical settling.

We find that the outcome of the dust migration process is very sensitive to the physical conditions within the disc. For high dust-to-gas ratios (greater than 0.01) or flattened disc structures (H/R 0.05), growing dust grains can efficiently decouple from the gas, leading to a high concentration of grains at a critical radius of a few AU. Decoupling of grains can occur at a large fraction (greater than 0.1) of the initial radius, for a dust-to-gas ratio larger than ~ 0.05. The exact value of the required dust-to-gas ratio for dust to stop its migration is strongly dependent on the disc temperature structure. Non growing dust grains are accreted for discs with flat surface density profiles (p less than 0.7) while they always remain in the disc if the surface density is steep enough (p less than 1.2). Both the presence of large grains and vertical settling tend to favour the accretion of non growing dust grains onto the central object, but it slows down the migration of growing dust grains. Importantly, all the disc configurations are found to have favourable temperature profiles over most of the disc to retain their planetesimals.