Particle accretion onto planets in discs with hydrodynamic turbulence

Abstract

The growth process of proto-planets can be sped-up by accreting a large number of solid, pebble-sized objects that are still present in the protoplanetary disc. It is still an open question on how efficient this process works in realistic turbulent discs. Here, we investigate the accretion of pebbles in turbulent discs that are driven by the purely hydrodynamical vertical shear instability (VSI). For this purpose, we perform global three-dimensional simulations of locally isothermal, VSI turbulent discs with embedded protoplanetary cores from 5 to 100 $M_\oplus$ that are placed at 5.2 au distance from the star. In addition, we follow the evolution of a swarm of embedded pebbles of different size under the action of drag forces between gas and particles in this turbulent flow. Simultaneously, we perform a set of comparison simulations for laminar viscous discs where the particles experience stochastic kicks. For both cases, we measure the accretion rate onto the cores as a function of core mass and Stokes number ($\tau_s$) of the particles and compare it to recent MRI turbulence simulations. Overall the dynamic is very similar for the particles in the VSI turbulent disc and the laminar case with stochastic kicks. For the small mass planets (i.e. 5 and 10 $M_\oplus$), well-coupled particles with $\tau_s = 1$, which have a size of about one meter at this location, we find an accretion efficiency (rate of particles accreted over drifting inward) of about 1.6-3%. For smaller and larger particles this efficiency is higher. However, the fast inward drift for $\tau_s = 1$ particles makes them the most effective for rapid growth, leading to mass doubling times of about 20,000 yr. For masses between 10 and 30 $M_\oplus$ the core reaches the pebble isolation mass and the particles are trapped at the pressure maximum just outside of the planet, shutting off further particle accretion.