On the survival of zombie vortices in protoplanetary discs

Abstract: Recently it has been proposed that the zombie vortex instability (ZVI) could precipitate hydrodynamical activity and angular momentum transport in unmagnetised regions of protoplanetary discs, also known as "dead zones". In this letter we scrutinise, with high resolution 3D spectral simulations, the onset and survival of this instability in the presence of viscous and thermal physics. First, we find that the ZVI is strongly dependent on the nature of the viscous operator. Although the ZVI is easily obtained with hyper-diffusion, it is difficult to sustain with physical (second order) diffusion operators up to Reynolds numbers as high as 10^7. This sensitivity is probably due to the ZVI's reliance on critical layers, whose characteristic lengthscale, structure, and dynamics are controlled by viscous diffusion. Second, we observe that the ZVI is sensitive to radiative processes, and indeed only operates when the Peclet number is greater than a critical value ~10^4, or when the cooling time is longer than ~10 Omega^{-1}. As a consequence, the ZVI struggles to appear at R>0.3 AU in standard 0.01 M_sun T Tauri disc models, though younger more massive disks provide a more hospitable environment. Together these results question the prevalence of the ZVI in protoplanetary discs.