Dust Trapping by Vortices in Transitional Disks: Evidence for Non-ideal MHD Effects in Protoplanetary Disks

TL;DR

This study uses 3D MHD simulations to demonstrate that non-ideal MHD effects like ambipolar diffusion enable vortex formation at planet-induced gaps in protoplanetary disks, facilitating dust trapping and explaining observed asymmetries.

Contribution

It provides the first detailed simulation evidence that ambipolar diffusion suppresses turbulence enough to allow vortex formation at gap edges, impacting dust trapping and observational signatures.

Findings

Vortices form at gap edges under ambipolar diffusion conditions.

Dust particles of various sizes are efficiently trapped by vortices.

Simulated ALMA images match observed disk asymmetries.

Abstract

We study particle trapping at the edge of a gap opened by a planet in a protoplanetary disk. In particular, we explore the effects of turbulence driven by the magnetorotational instability on particle trapping, using global three-dimensional magnetohydrodynamic (MHD) simulations including Lagrangian dust particles. We study disks either in the ideal MHD limit or dominated by ambipolar diffusion (AD) that plays an essential role at the outer regions of a protoplanetary disk. With ideal MHD, strong turbulence (the equivalent viscosity parameter $\alpha\sim 10^{-2}$) in disks prevents vortex formation at the edge of the gap opened by a 9 $M_{J}$ planet, and most particles (except the particles that drift fastest) pile up at the outer gap edge almost axisymmetrically. When AD is considered, turbulence is significantly suppressed ($\alpha\lesssim 10^{-3}$), and a large vortex forms at the edge of the planet induced gap, which survives $\sim$ 1000 orbits. The vortex can efficiently trap dust particles that span 3 orders of magnitude in size within 100 planetary orbits. We have also carried out two-dimensional hydrodynamical simulations using viscosity as an approximation to MHD turbulence. These hydrodynamical simulations can reproduce vortex generation at the gap edge as seen in MHD simulations. Finally, we use our simulation results to generate synthetic images for ALMA dust continuum observations on Oph IRS 48 and HD 142527, which show good agreement with existing observations. Predictions for future ALMA cycle 2 observations have been made. We conclude that the asymmetry in ALMA observations can be explained by dust trapping vortices and the existence of vortices could be the evidence that the outer protoplanetary disks are dominated by ambipolar diffusion with $\alpha<10^{-3}$ at the disk midplane.