Dust Dynamics in Protoplanetary Disk Winds Driven by Magnetorotational Turbulence: A Mechanism for Floating Dust Grains With Characteristic Sizes

Abstract

We investigate the dynamics of dust grains of various sizes in protoplanetary disk winds driven by magnetorotational turbulence, by simulating the time evolution of the dust grain distribution in the vertical direction. Small dust grains, which are well-coupled to the gas, are dragged upward with the upflowing gas, while large grains remain near the midplane of a disk. Intermediate-size grains float near the sonic point of the disk wind located at several scale heights from the midplane, where the grains are loosely coupled to the background gas. For the minimum mass solar nebula at 1 au, dust grains with size of 25-45 μm float around 4 scale heights from the midplane. Considering the dependence on the distance from the central star, smaller-size grains remain only in an outer region of the disk, while larger-size grains are distributed in a broader region. We also discuss the implications of our result for observations of dusty material around young stellar objects.

On the maximum grain size entrained by photoevaporative winds

“…An overdensity occurs near the disc surface whenever there is a local minimum in the velocity profile, a result of the velocity-density relation in Equation (7). A similar result was found by Miyake et al (2016) in magnetically-driven winds. Beyond zc where aerodynamic drag takes over as the dominant force, the larger grains experience a greater acceleration due to their large differential velocities with the gas.…”

Dynamics of magnetic flux tubes in accretion discs of T Tauri stars

“…We propose that such this process can be responsible for the variations in circumstellar extinction observed in T Tauri stars. Similar hypothesis was proposed by Miyake et al (2016), who simulated the time evolution of the dust grain distribution in the vertical direction inside the minimum mass solar nebula.…”

The dispersal of planet-forming discs: theory confronts observations

“…These rather short timescales, which are clearly in contrast with disc observations, do not account for global viscous accretion, which would slow down the process. Very recent work expands on these results to present one-dimensional models of disc evolution including the effects of viscous heating, in addition to the loss of mass and angular moment by the disc wind [ 132 ]. The focus of this work is more on the early stages of evolution, when accretion heating is important and can give rise to density structures which show a positive radial slope for the surface density in the inner disc regions.…”