Dust Settling and Clumping in MRI-turbulent Outer Protoplanetary Disks

Ziyan Xu,Xue-Ning Bai

doi:10.3847/1538-4357/ac31a7

Abstract

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 nonideal MHD effect, applicable to outer disk regions. We first show that dust backreaction enhances dust settling toward 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.

Highlights

The process of planet formation encompasses a growth of more than 13 orders of magnitude in size
Particle clumping under the conventional streaming instability (SI) scenario is expected to be triggered in the presence of a weak background pressure gradient, while dust clumping under magneto-rotational instability (MRI) turbulence occurs in the presence of a local pressure maximum, where the pressure variation induced by zonal flow overcomes the background pressure gradient
We target disk outer regions with turbulence driven by the MRI, incorporating ambipolar diffusion (AD) as the dominant nonideal MHD effect, which provides a relatively low but realistic turbulent environment

Summary

Introduction

The process of planet formation encompasses a growth of more than 13 orders of magnitude in size. Micron-sized dust particles stick together and coagulate into mm to cm sized solids. Km-sized planetesimals grow into planetary cores, and eventually into terrestrial or giant planets of up to 105 km in size. The intermediate stage involves the formation of km-sized planetesimals out of from mm-cm sized particles, and it is perhaps the least understood problem in planet formation (e.g., see review by Chiang & Youdin (2010)). The obstacle in planetesimal formation arises primarily because of the existence of growth barriers. Direct growth of dust beyond ∼mm-cm size has been found to be difficult, and encounters fragmentation and

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