Global three-dimensional simulations of outer protoplanetary discs with ambipolar diffusion

Can Cui,Xue-Ning Bai

doi:10.1093/mnras/stab2220

Abstract

ABSTRACT The structure and evolution of protoplanetary discs (PPDs) are largely governed by disc angular momentum transport, mediated by magnetic fields. In the most observable outer disc, ambipolar diffusion is the primary non-ideal magnetohydrodynamic (MHD) effect. In this work, we study the gas dynamics in outer PPDs by conducting a series of global three-dimensional non-ideal MHD simulations with ambipolar diffusion and net poloidal magnetic flux, using the Athena++ MHD code, with resolution comparable to local simulations. Our simulations demonstrate the coexistence of magnetized disc winds and turbulence driven by the magneto-rotational instability (MRI). While MHD winds dominate disc angular momentum transport, the MRI turbulence also contributes significantly. We observe that magnetic flux spontaneously concentrates into axisymmetric flux sheets, leading to radial variations in turbulence levels, stresses, and accretion rates. Annular substructures arise as a natural consequence of magnetic flux concentration. The flux concentration phenomena show diverse properties with different levels of disc magnetization and ambipolar diffusion. The disc generally loses magnetic flux over time, though flux sheets could prevent the leak of magnetic flux in some cases. Our results demonstrate the ubiquity of disc annular substructures in weakly MRI turbulent outer PPDs and imply a stochastic nature of disc evolution.

Highlights

Protoplanetary disks (PPDs) are the birth place of planets, where dust and ice particles coagulate into planetesimals, seeding further growth into terrestrial planets and cores of gas giants
Three dimensions and resolution – We emphasize that conducting 3D simulations with sufficiently high resolution to resolve the magneto-rotational instability (MRI) turbulence is essential to properly characterize the gas dynamics of the outer PPDs. 2D simulations are experimented utilizing same resolution, and the results indicate that the system is steady with no evidence of MRI for the long term evolution
As we focus on the gas dynamics in the outer PPDs, regions inside the inner boundary corresponds to the inner disk, which is expected to be threaded by poloidal magnetic flux and launches its own disk winds

Summary

Introduction

Protoplanetary disks (PPDs) are the birth place of planets, where dust and ice particles coagulate into planetesimals, seeding further growth into terrestrial planets and cores of gas giants. The Atacama Large Millimiter/submillimeter Array (ALMA) has been revolutionary that uncovers a diverse range of disk substructures (ALMA Partnership et al 2015; Andrews 2020), annular substructures (e.g Isella et al 2016; Andrews et al 2016; van Boekel et al 2017; Avenhaus et al 2018; Long et al 2018; Huang et al 2018a), spirals (e.g Benisty et al 2015; Perez et al 2016; Huang et al 2018b), and crescent-shaped azimuthal asymmetries (e.g van der Marel et al 2013, 2020) These features have triggered extensive research on their origins. They could be interpreted as signposts of unseen planets embedded in the disk, reflecting planet formation processes caught in action, while they may reflect local concentration of dust that could potentially become precursors to planet formation

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