Weak Turbulence in the HD 163296 Protoplanetary Disk Revealed by ALMA CO Observations

TL;DR

This study uses ALMA observations of CO lines to place tight constraints on turbulence in the outer disk of HD 163296, challenging existing theories of magneto-rotational instability-driven turbulence.

Contribution

It provides the first direct, multi-line observational limits on turbulence in the outer regions of a protoplanetary disk, revealing turbulence levels much lower than predicted by models.

Findings

Turbulence in the upper layers of the outer disk is less than 3% of the local sound speed.

Observed turbulence levels are about an order of magnitude lower than MHD turbulence predictions.

Results suggest MRI-driven turbulence may be less effective in outer disk regions.

Abstract

Turbulence can transport angular momentum in protoplanetary disks and influence the growth and evolution of planets. With spatially and spectrally resolved molecular emission line measurements provided by (sub)millimeter interferometric observations, it is possible to directly measure non-thermal motions in the disk gas that can be attributed to this turbulence. We report a new constraint on the turbulence in the disk around HD 163296, a nearby young A star, determined from ALMA Science Verification observations of four CO emission lines (the CO(3-2), CO(2-1), 13CO(2-1), and C18O(2-1) transitions). The different optical depths for these lines permit probes of non-thermal line-widths at a range of physical conditions (temperature and density) and depths into the disk interior. We derive stringent limits on the non-thermal motions in the upper layers of the outer disk such that any contribution to the line-widths from turbulence is <3% of the local sound speed. These limits are approximately an order of magnitude lower than theoretical predictions for full-blown MHD turbulence driven by the magneto-rotational instability, potentially suggesting that this mechanism is less efficient in the outer (R>30AU) disk than has been previously considered.