Gravito-Turbulent Disks in 3D: Turbulent Velocities vs. Depth

TL;DR

This study uses 3D hydrodynamic simulations to show that gravito-turbulence in protoplanetary disks is nearly uniform with height, contrasting with MRI turbulence, which increases significantly towards the surface, aiding in identifying turbulence sources.

Contribution

The paper demonstrates that gravito-turbulence maintains similar velocities throughout the disk height, providing a new diagnostic to distinguish turbulence mechanisms in protoplanetary disks.

Findings

Gravito-turbulence velocities are nearly uniform vertically.

Turbulent velocities increase only by a factor of 2 from midplane to surface.

MRI turbulence velocities increase at least 15 times from midplane to surface.

Abstract

Characterizing turbulence in protoplanetary disks is crucial for understanding how they accrete and spawn planets. Recent measurements of spectral line broadening promise to diagnose turbulence, with different lines probing different depths. We use 3D local hydrodynamic simulations of cooling, self-gravitating disks to resolve how motions driven by "gravito-turbulence" vary with height. We find that gravito-turbulence is practically as vigorous at altitude as at depth: even though gas at altitude is much too rarefied to be itself self-gravitating, it is strongly forced by self-gravitating overdensities at the midplane. The long-range nature of gravity means that turbulent velocities are nearly uniform vertically, increasing by just a factor of 2 from midplane to surface, even as the density ranges over nearly three orders of magnitude. The insensitivity of gravito-turbulence to height contrasts with the behavior of disks afflicted by the magnetorotational instability (MRI); in the latter case, non-circular velocities increase by at least a factor of 15 from midplane to surface, with various non-ideal effects only magnifying this factor. The distinct vertical profiles of gravito-turbulence vs. MRI turbulence may be used in conjunction with measurements of non-thermal linewidths at various depths to identify the source of transport in protoplanetary disks.