Zombie Vortex Instability. III. Persistence with Nonuniform Stratification and Radiative Damping

Abstract

The Zombie Vortex Instability (ZVI) occurs in the dead zones of protoplanetary disks (PPDs) where perturbations excite baroclinic critical layers, generating "zombie" vortices and turbulence. In this work, we investigate ZVI with nonuniform vertical stratification; while ZVI is triggered in the stratified regions away from the midplane, the subsequent turbulence propagates into and fills the midplane. ZVI turbulence alters the background Keplerian shear flow, creating a steady-state zonal flow. Intermittency is observed, where the flow cycles through near-laminar phases of zonal flow punctuated by chaotic bursts of new vortices. ZVI persists in the presence of radiative damping, as long as the thermal relaxation timescale is more than a few orbital periods. We refute the premature claim by Lesur & Latter (2016) that radiative damping inhibits ZVI for disk radii r>0.3 au. Their conclusions were based on unrealistically short cooling times using opacities with virtually no grain growth. We explore different grain growth and vertical settling scenarios, and find that the gas and dust in off-midplane regions are not necessarily in local thermodynamic equilibrium (LTE) with each other. In such cases, thermal relaxation timescales can be orders of magnitude longer than the optically thin cooling times assuming LTE because of the finite time for energy to be exchanged between gas and dust grains via collisions. With minimal amounts of grain growth and dust settling, the off-midplane regions of disks are susceptible to ZVI and much of the planet-forming regions can be filled with zombie vortices and turbulence.