Empirical Constraints on Turbulence in Protoplanetary Accretion Disks

TL;DR

This study uses high-resolution submillimeter observations to constrain turbulence levels in protoplanetary disks, finding predominantly low turbulence consistent with theoretical models, and providing the first direct observational limits on turbulent linewidths.

Contribution

It provides the first direct observational constraints on turbulence in protoplanetary disks using high-resolution CO line data and physical disk models.

Findings

TW Hya has turbulence <40 m/s, about 10% of sound speed.

HD 163296 shows tentative turbulence detection around 300 m/s.

Results support subsonic turbulence predictions with alpha ~ 0.01.

Abstract

We present arcsecond-scale Submillimeter Array observations of the CO(3-2) line emission from the disks around the young stars HD 163296 and TW Hya at a spectral resolution of 44 m/s. These observations probe below the ~100 m/s turbulent linewidth inferred from lower-resolution observations, and allow us to place constraints on the turbulent linewidth in the disk atmospheres. We reproduce the observed CO(3-2) emission using two physical models of disk structure: (1) a power-law temperature distribution with a tapered density distribution following a simple functional form for an evolving accretion disk, and (2) the radiative transfer models developed by D'Alessio et al. that can reproduce the dust emission probed by the spectral energy distribution. Both types of models yield a low upper limit on the turbulent linewidth (Doppler b-parameter) in the TW Hya system (<40 m/s), and a tentative (3-sigma) detection of a ~300 m/s turbulent linewidth in the upper layers of the HD 163296 disk. These correspond to roughly <10% and 40% of the sound speed at size scales commensurate with the resolution of the data. The derived linewidths imply a turbulent viscosity coefficient, alpha, of order 0.01 and provide observational support for theoretical predictions of subsonic turbulence in protoplanetary accretion disks.