The Dead Zones of Protoplanetary Disks are Not Dead

Abstract

We show that the “dead” zone of a protoplanetary disk fills with robust 3D vortices from a purely hydrodynamic instability. This new instability is not linear and requires a weak finite-amplitude initial perturbation. The instability was not seen previously either due to a lack of numerical spatial resolution, or because many previous simulations either ignored vertical gravity or had initial flows with constant density. Our new finite-amplitude instability is due to a family of previously-unknown critical layers that form in rotating, shearing, vertically stratified flows like those in protoplanetary disks. Initial perturbations of white noise (with Mach numbers much less than unity), waves, or vortices can trigger the instability. A small-volume, small-amplitude initial vortex confined to one part of the disk can fill the disk with vortices by exciting a nearby critical layer. The critical layer produces an intense vortex layer that rolls-up to form vortices with large-amplitudes and volumes. This 1st generation of vortices then sheds waves that excite nearby critical layers, which in turn, create a 2nd generation of vortices with large amplitudes and volumes. The mechanism of exciting nearby critical layers and turning them into large vortices self-similarly, self-replicates until large vortices fill the disk at all radii.