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The growth and migration of planetesimals in a young protoplanetary disc arefundamental to planet formation. In all models of early growth, there areseveral 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 solidsfrom beyond their gravitationally focused cross-section. This gas drag-enhancedaccretion can allow planetesimals to grow at rapid rates, in principle. Wepresent self-consistent hydrodynamics simulations with direct particleintegration and gas drag coupling to estimate the rate of planetesimal growthdue to pebble accretion. Wind tunnel simulations are used to explore a range ofparticle sizes and disc conditions. We also explore analytic estimates ofplanetesimal growth and numerically integrate planetesimal drift due to theaccretion of small solids. Our results show that, for almost every case that weconsider, there is a clearly preferred particle size for accretion that dependson the properties of the accreting planetesimal and the local disc conditions.For solids much smaller than the preferred particle size, accretion rates aresignificantly reduced as the particles are entrained in the gas and flow aroundthe planetesimal. Solids much larger than the preferred size accrete at ratesconsistent with gravitational focusing. Our analytic estimates for pebbleaccretion highlight the timescales that are needed for the growth of largeobjects under different disc conditions and initial planetesimal sizes.

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