Weak Turbulence in the HD 163296 Protoplanetary Disk

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

Authors:

Flaherty et al

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 less than 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.

Turbulent Wakes Around High Mass Exoplanets/Brown Dwarfs can Destroy Protoplanetary Disk Gaps

TURBULENT WAKES DESTROY GAPS AROUND HIGH-MASS PLANETS OR BROWN DWARFS IN RADIATIVELY INEFFICIENT DISKS

Authors:

Richert et al

Abstract:

Recent observations of gaps and non-axisymmetric features in the dust distributions of transition disks have been interpreted as evidence of embedded massive protoplanets. However, comparing the predictions of planet–disk interaction models to the observed features has shown far from perfect agreement. This may be due to the strong approximations used for the predictions. For example, spiral arm fitting typically uses results that are based on low-mass planets in an isothermal gas. In this work, we describe two-dimensional, global, hydrodynamical simulations of disks with embedded protoplanets, with and without the assumption of local isothermality, for a range of planet-to-star mass ratios 1–10 ${{M}_{{\rm J}}}$ for a 1 ${{M}_{\odot }}$ star. We use the Pencil Code in polar coordinates for our models. We find that the inner and outer spiral wakes of massive protoplanets ($M\gtrsim 5\;{{M}_{{\rm J}}}$) produce significant shock heating that can trigger buoyant instabilities. These drive sustained turbulence throughout the disk when they occur. The strength of this effect depends strongly on the mass of the planet and the thermal relaxation timescale; for a $10\;{{M}_{{\rm J}}}$ planet embedded in a thin, purely adiabatic disk, the spirals, gaps, and vortices typically associated with planet–disk interactions are disrupted. We find that the effect is only weakly dependent on the initial radial temperature profile. The spirals that form in disks heated by the effects we have described may fit the spiral structures observed in transition disks better than the spirals predicted by linear isothermal theory.