Particle dynamics in discs with turbulence generated by the vertical shear instability

TL;DR

This study uses 3D simulations to show that vertical shear instability (VSI) in protoplanetary discs can generate turbulence, concentrate particles, and facilitate early planet formation through overdensities and low collision velocities.

Contribution

It demonstrates that VSI-driven turbulence effectively concentrates particles and promotes conditions favorable for planetesimal formation in realistic, irradiated protoplanetary discs.

Findings

VSI generates turbulence in full 3D protoplanetary discs.

Particles show overdensities up to 5 times the average.

Low relative velocities facilitate particle growth.

Abstract

Among the candidates for generating turbulence in accretion discs in situations with low intrinsic ionization the vertical shear instability (VSI) has become an interesting candidate, as it relies purely on a vertical gradient in the angular velocity. Existing simulations have shown that $\alpha$-values a few times $10^{-4}$ can be generated. The particle growth in the early planet formation phase is determined by the dynamics of dust particles. Here, we address in particular the efficiency of VSI-turbulence in concentrating particles in order to generate overdensities and low collision velocities. We perform 3D numerical hydrodynamical simulations of accretion discs around young stars that include radiative transport and irradiation from the central star. The motion of particles within a size range of a fraction of mm up to several m is followed using standard drag formula. We confirm that under realistic conditions the VSI is able to generate turbulence in full 3D protoplanetary discs. The irradiated disc shows turbulence within 10 to 60au. The mean radial motion of the gas is such that it is directed inward near the midplane and outward in the surface layers. We find that large particles drift inward with the expected speed, while small particles can experience phases of outward drift. Additionally, the particles show bunching behaviour with overdensities reaching 5 times the average value, which is strongest for dimensionless stopping times around unity. Particles in a VSI-turbulent discs are concentrated in large scale turbulent eddies and show low relative speeds that allow for growing collisions. The reached overdensities will also allow for the onset streaming instabilities further enhancing particle growth. The outward drift for small particles at higher disk elevations allows for the transport of processed high temperature material in the Solar System to larger distances.