Simulations of Small Solid Accretion onto Planetesimals in the Presence of Gas

TL;DR

This study uses hydrodynamics simulations and analytic models to investigate how small solids are accreted onto planetesimals in protoplanetary discs, revealing a preferred particle size for efficient growth influenced by disc conditions.

Contribution

The paper provides a comprehensive simulation and analytic framework for understanding pebble accretion onto planetesimals, highlighting the role of particle size and disc environment in growth rates.

Findings

Identified a preferred particle size for efficient accretion.

Showed that smaller particles are entrained in gas flow, reducing accretion.

Large particles accrete primarily through gravitational focusing.

Abstract

The growth and migration of planetesimals in a young protoplanetary disc are fundamental to planet formation. In all models of early growth, there are several processes that can inhibit grains from reaching larger sizes. Nevertheless, observations suggest that growth of planetesimals must be rapid. If a small number of 100 km sized planetesimals do manage to form in the disc, then gas drag effects could enable them to efficiently accrete small solids from beyond their gravitationally focused cross-section. This gas drag-enhanced accretion can allow planetesimals to grow at rapid rates, in principle. We present self-consistent hydrodynamics simulations with direct particle integration and gas drag coupling to estimate the rate of planetesimal growth due to pebble accretion. Wind tunnel simulations are used to explore a range of particle sizes and disc conditions. We also explore analytic estimates of planetesimal growth and numerically integrate planetesimal drift due to the accretion of small solids. Our results show that, for almost every case that we consider, there is a clearly preferred particle size for accretion that depends on the properties of the accreting planetesimal and the local disc conditions. For solids much smaller than the preferred particle size, accretion rates are significantly reduced as the particles are entrained in the gas and flow around the planetesimal. Solids much larger than the preferred size accrete at rates consistent with gravitational focusing. Our analytic estimates for pebble accretion highlight the timescales that are needed for the growth of large objects under different disc conditions and initial planetesimal sizes.