Stratified and vertically-shearing streaming instabilities in protoplanetary disks

TL;DR

This paper extends linear stability analysis of streaming instabilities in protoplanetary disks to include vertical stratification, revealing a dominant vertically-shearing instability that grows rapidly and may significantly influence planetesimal formation.

Contribution

It introduces a vertically-global linear stability analysis, identifying a new dominant vertically-shearing streaming instability in stratified dust layers.

Findings

Vertically-shearing streaming instabilities grow on orbital timescales.

These instabilities occur on radial scales of about 10^{-3} times the gas scale height.

Classic radial-drift streaming instability has smaller growth rates and larger scales.

Abstract

Under the right conditions, the streaming instability between imperfectly coupled dust and gas is a powerful mechanism for planetesimal formation as it can concentrate dust grains to the point of gravitational collapse. In its simplest form, the streaming instability can be captured by analyzing the linear stability of unstratified disk models, which represent the midplane of protoplanetary disks. We extend such studies by carrying out vertically-global linear stability analyses of dust layers in protoplanetary disks. We find the dominant form of instability in stratified dust layers is one driven by the vertical gradient in the rotation velocity of the dust-gas mixture, but also requires partial dust-gas coupling. These vertically-shearing streaming instabilities grow on orbital timescales and occur on radial length scales $\sim10^{-3}H_\mathrm{g}$, where $H_\mathrm{g}$ is the local pressure scale height. The classic streaming instability, associated with the relative radial drift between dust and gas, occur on radial length scales $\sim10^{-2}H_\mathrm{g}$, but have much smaller growth rates than vertically-shearing streaming instabilities. Including gas viscosity is strongly stabilizing and leads to vertically-elongated disturbances. We briefly discuss the potential effects of vertically-shearing streaming instabilities on planetesimal formation.