Dust Trapping by Vortices in Protoplanetary Disks

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

Authors:

Zhu et al

Abstract:

We perform a systematic study of particle trapping at the edge of a gap opened by a planet in a protoplanetary disk. In particular, we study 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 α∼10−2) in disks prevents vortex formation at the edge of the gap opened by a 9 MJ 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 (α≲10−3), and a large vortex forms at the edge of the planet induced gap. The vortex can efficiently trap dust particles that span 3 orders of magnitude in size within 100 planetary orbits, producing more than a factor of 10 enhancement in the dust surface density within the vortex. The presence of the vortex also makes the gap edge appear eccentric. The vortex in MHD simulations with AD survives ∼ 1000 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. If the disk asymmetry in recent ALMA observations is indeed due to dust trapping in a vortex at the planet-induced gap edge, we conclude that this asymmetry may be the evidence that the outer protoplanetary disks are dominated by ambipolar diffusion, as suggested by disk ionization calculations, and the equivalent α in the outer disk is less than 10−3.