Substructures Induced by Dust Drag in Protoplanetary Disks

TL;DR

This study uses 2D simulations to show how dust-gas interactions, including streaming and vertical shear instabilities, can produce complex dust structures in protoplanetary disks, potentially aiding planetesimal formation.

Contribution

It reveals that intrinsic gas-dust interactions can create prominent dust substructures without embedded planets, influenced by turbulence and instabilities.

Findings

Dust substructures can form due to nonlinear streaming instability.

Vigorous vertical shear instability enhances dust mixing and turbulence.

Dust-to-gas ratios up to 20-50 suggest conditions favorable for planetesimal formation.

Abstract

Dust substructures observed in protoplanetary disks are commonly attributed to embedded planets; however, intrinsic gas-dust interactions can also generate complex morphologies. We performed two-dimensional, axisymmetric simulations of gas and dust that include dust back-reaction and parameterized turbulence to investigate how the streaming instability (SI) and vertical shear instability (VSI) shape dust distributions. With moderate viscosity and sufficiently high metallicity, we identify a characteristic shuttlecock-shaped dust substructure composed of a dense, vertically settled "head" and a vertically extended "tail." This morphology arises from nonlinear SI driven by marginally coupled grains and the associated modification of gas flows. The dust scale height in the tail exceeds predictions based on the simple diffusion-settling balance, indicating strong self-generated turbulence. With lower viscosity, VSI becomes more vigorous, disrupts midplane structures, and increases vertical stirring; nevertheless, for dust grains with Stokes numbers around 0.01, SI can still attain dust-to-gas ratios of up to 20-50, potentially approaching the Hill density for gravitational binding. Our results demonstrate that intrinsic gas-dust interactions can generate prominent dust substructures even in disks with finite viscosity and, under favorable conditions, concentrate dust to levels relevant for planetesimal formation.