Dust Settling and Clumping in MRI Turbulent Outer Protoplanetary Disks

TL;DR

This study demonstrates that dust can effectively settle and clump in MRI-turbulent outer protoplanetary disks, supporting planetesimal formation through mechanisms involving dust backreaction and pressure maxima.

Contribution

It provides high-resolution MHD simulations showing dust clumping is easier in MRI turbulence than previously thought, highlighting the roles of dust backreaction and zonal flows.

Findings

Dust backreaction enhances dust settling.

Lower threshold of solid abundance needed for clumping.

MRI zonal flows create pressure maxima aiding clumping.

Abstract

Planetesimal formation is a crucial yet poorly understood process in planet formation. It is widely believed that planetesimal formation is the outcome of dust clumping by the streaming instability (SI). However, recent analytical and numerical studies have shown that the SI can be damped or suppressed by external turbulence, and at least the outer regions of protoplanetary disks are likely weakly turbulent due to magneto-rotational instability (MRI). We conduct high-resolution local shearing-box simulations of hybrid particle-gas magnetohydrodynamics (MHD), incorporating ambipolar diffusion as the dominant non-ideal MHD effect, applicable to outer disk regions. We first show that dust backreaction enhances dust settling towards the midplane by reducing turbulence correlation time. Under modest level of MRI turbulence, we find that dust clumping is in fact easier than the conventional SI case, in the sense that the threshold of solid abundance for clumping is lower. The key to dust clumping includes dust backreaction and the presence of local pressure maxima, which in our work is formed by the MRI zonal flows overcoming background pressure gradient. Overall, our results support planetesimal formation in the MRI-turbulent outer protoplanetary disks, especially in ring-like substructures.