Origin of Weak Turbulence in the Outer Regions of Protoplanetary Disks

TL;DR

This study investigates the origin of weak turbulence in the outer regions of protoplanetary disks, suggesting that low ionization and magnetic field strength can suppress turbulence, aligning with recent ALMA observations.

Contribution

The paper demonstrates through simulations that weak turbulence in outer disks arises under low ionization and magnetic field conditions, challenging previous assumptions about MRI-driven turbulence.

Findings

Turbulence persists even with strong laminar winds due to localized MRI activity.

Weak magnetic fields and low ionization levels can suppress turbulence below observational limits.

Outer disk ionization is likely reduced, leading to weak turbulence and magnetic fields.

Abstract

The mechanism behind angular momentum transport in protoplanetary disks, and whether this transport is turbulent in nature, is a fundamental issue in planet formation studies. Recent ALMA observations have suggested that turbulent velocities in the outer regions of these disks are less than ~5-10% of the sound speed, contradicting theoretical predictions of turbulence driven by the magnetorotational instability (MRI). These observations have generally been interpreted to be consistent with a large-scale laminar magnetic wind driving accretion. Here, we carry out local, shearing box simulations with varying ionization levels and background magnetic field strengths in order to determine which parameters produce results consistent with observations. We find that even when the background magnetic field launches a strong largely laminar wind, significant turbulence persists and is driven by localized regions of vertical magnetic field (the result of zonal flows) that are unstable to the MRI. The only conditions for which we find turbulent velocities below the observational limits are weak background magnetic fields and ionization levels well below that usually assumed in theoretical studies. We interpret these findings within the context of a preliminary model in which a large scale magnetic field, confined to the inner disk, hinders ionizing sources from reaching large radial distances, e.g., through a sufficiently dense wind. Thus, in addition to such a wind, this model predicts that for disks with weakly turbulent outer regions, the outer disk will have significantly reduced ionization levels compared to standard models and will harbor only a weak vertical magnetic field.