Toroidal vortices and the conglomeration of dust into rings in protoplanetary discs

TL;DR

This paper introduces a hydrodynamical instability driven by dust settling in protoplanetary discs, leading to the formation of observable dust rings and potentially impacting planet formation processes.

Contribution

It identifies a novel baroclinic instability caused by dust-to-gas ratio variations, explaining ring formation without requiring protoplanets.

Findings

Dust rings can form due to the instability, matching observations.

The instability influences dust concentration and migration.

It may affect angular momentum transport in discs.

Abstract

We identify a new hydrodynamical instability in protoplanetary discs that may arise due to variations in the dust-to-gas ratio and may lead to concentration of dust grains within a disc. The instability can arise due to dust settling, which produces a vertical compositional entropy gradient. The entropy gradient drives a baroclinic instability that is capable of creating toroidal gas vortices that gather dust into rings. Such dust rings are potentially observable via continuum emission of the dust or scattered light. Indeed, this instability may offer an explanation for the rings recently observed in the discs around the young stars HL Tau and TW Hya that does not rely on clearing by protoplanets. The instability may also have wider ramifications, potentially aiding dust agglomeration, altering the radial migration of larger planetesimals, and modifying angular momentum transport within a disc.