Turbulent Transport of Dust Particles in Protostellar Disks: The Effect of Upstream Diffusion

TL;DR

This study investigates how turbulence-driven diffusion influences the radial transport of dust in protostellar disks, revealing significant upstream diffusion that affects dust distribution and composition in early planetary systems.

Contribution

It provides the first detailed measurement of diffusion coefficients from 3D simulations, highlighting the role of turbulence in dust transport and distribution in different disk conditions.

Findings

High radial diffusion coefficient in gravitationally unstable disks

Upstream diffusion effectively transports micron to mm-sized grains outward

Diffusion varies with disk turbulence type and vertical structure

Abstract

We study the long-term radial transport of micron to mm-size grain in protostellar disks (PSDs) based on diffusion and viscosity coefficients measured from 3D global stratified-disk simulations with a Lagrangian hydrodynamic method. While gas-drag tend to transport dust species radially inwards, stochastic diffusion can spread a considerable fraction of dust radially outwards (upstream) depending on the nature of turbulence. In gravitationally unstable disks, we measure a high radial diffusion coefficient Dr with little dependence on altitude. This leads to strong and vertically homogeneous upstream diffusion in early PSDs. In the solar nebula, the robust upstream diffusion of micron to mm size grains not only efficiently transports highly refractory mocron-size grains (such as those identified in the samples of comet 81P/Wild 2) from their regions of formation inside the snow line out to the Kuiper Belt, but can also spread mm-size CAI formed in the stellar proximity to distances where they can be assimilated into chondritic meteorites. In disks dominated by magnetorotational instability (MRI), the upstream diffusion effect is generally milder, with a separating feature due to diffusion being stronger in the surface layer than the midplane. This variation becomes much more pronounced if we additionally consider a quiescent midplane with lower turbulence and larger characteristic dust size due to non-ideal MHD effects. This segregation scenario helps to account for dichotomy of two dust populations' spatial distribution as observed in scattered light and ALMA images.