Kinematics of solid particles in a turbulent protoplanetary disc

TL;DR

This study uses numerical simulations to analyze how magnetorotational turbulence affects the motion and distribution of solid particles in a protoplanetary disc, revealing anisotropic velocity dispersions and transient particle clumping.

Contribution

It provides new insights into particle velocity dispersions, spatial distributions, and stability of clumps in turbulent protoplanetary discs, with detailed numerical analysis.

Findings

Velocity dispersions depend on particle size and turbulence anisotropy.

Relative speeds between particles decrease with separation, especially for similar sizes.

Particle clumps are short-lived and resist gravitational collapse.

Abstract

We perform numerical simulations of solid particle motion in a shearing box model of a protoplanetary disc. The accretion flow is turbulent due to the action of the magnetorotational instability. Aerodynamic drag on the particles is modelled using the Epstein law with the gas velocity interpolated to the particle position. The effect of the magnetohydrodynamic turbulence on particle velocity dispersions is quantified for solids of different stopping times t_s, or equivalently, different sizes. The anisotropy of the turbulence is reflected upon the dispersions of the particle velocity components, with the radial component larger than both the azimuthal and vertical components for particles larger than ~ 10 cm (assuming minimum-mass solar nebula conditions at 5 AU). The dispersion of the particle velocity magnitude, as well as that of the radial and azimuthal components, as functions of stopping time, agree with previous analytical results for isotropic turbulence. The relative speed between pairs of particles with the same value of t_s decays faster with decreasing separation than in the case of solids with different stopping time. Correlations in the particle number density introduce a non-uniform spatial distribution of solids in the 10 to 100 cm size range. Any clump of particles is disrupted by the turbulence in less than one tenth on an orbital period, and the maximally concentrated clumps are stable against self-gravitational collapse.