Streaming Instabilities in Accreting Protoplanetary Disks: A Parameter Study

The streaming instability (SI) is currently the leading candidate for triggering planetesimal formation in protoplanetary disks. Recently, a novel variation, the "azimuthal-drift" streaming instability (AdSI), was discovered in disks exhibiting laminar gas accretion. Unlike the classical SI, the AdSI does not require pressure gradients and can concentrate dust even at low abundances. We extend previous simulations of the AdSI to explore the impact of dust abundance, accretion-flow strength, pressure gradients, and grain size. For a dimensionless accretion-flow strength α _M = 0.1 and particle Stokes number St = 0.1, we find the AdSI produces dust filaments for initial dust-to-gas ratios as low as ϵ = 0.01. For ϵ ≳ 1, maximum dust-to-gas ratios of order 100 are attained, which can be expected to undergo gravitational collapse. Furthermore, even in systems dominated by the classical SI, an accretion flow drives filament formation, without which the disk remains in a state of small-scale turbulence. Our results suggest that an underlying accretion flow facilitates dust concentration and may thus promote planetesimal formation.