Zombie Vortex Instability I: A Purely Hydrodynamic Instability to Resurrect the Dead Zones of Protoplanetary Disks

TL;DR

This paper introduces Zombie Vortex Instability (ZVI), a hydrodynamic instability in stratified protoplanetary disk flows, which can generate sustained turbulence and vortices, potentially impacting planet formation processes.

Contribution

The study demonstrates that vertical stratification can induce ZVI, a previously unrecognized instability, in rotating shear flows, challenging the assumption that stable stratification always stabilizes such flows.

Findings

ZVI produces space-filling, sustained turbulence with large vortices.

Vertical stratification can destabilize flows via baroclinic critical layers.

ZVI is robust and occurs without special boundary condition tuning.

Abstract

There is considerable interest in hydrodynamic instabilities in dead zones of protoplanetary disks as a mechanism for driving angular momentum transport and as a source of particle-trapping vortices to mix chondrules and incubate planetesimal formation. We present simulations with a pseudo-spectral anelastic code and with the compressible code Athena, showing that stably stratified flows in a shearing, rotating box are violently unstable and produce space-filling, sustained turbulence dominated by large vortices with Rossby numbers of order 0.2-0.3. This Zombie Vortex Instability (ZVI) is observed in both codes and is triggered by Kolmogorov turbulence with Mach numbers less than 0.01. It is a common view that if a given constant density flow is stable, then stable vertical stratification should make the flow even more stable. Yet, we show that sufficient vertical stratification can be unstable to ZVI. ZVI is robust and requires no special tuning of boundary conditions, or initial radial entropy or vortensity gradients (though we have studied ZVI only in the limit of infinite cooling time). The resolution of this paradox is that stable stratification allows for a new avenue to instability: baroclinic critical layers. ZVI has not been seen in previous studies of flows in rotating, shearing boxes because those calculations frequently lacked vertical density stratification and/or sufficient numerical resolution. Although we do not expect appreciable angular momentum transport from ZVI in the small domains in this study, we hypothesize that ZVI in larger domains with compressible equations may lead to angular transport via spiral density waves.