On the survival of zombie vortices in protoplanetary discs

TL;DR

This study investigates the conditions under which the zombie vortex instability (ZVI) can occur and persist in protoplanetary discs, highlighting its dependence on viscous and thermal physics, and questioning its commonality.

Contribution

It provides a detailed analysis of how viscous and radiative processes influence the survival of ZVI in protoplanetary discs, challenging previous assumptions about its ubiquity.

Findings

ZVI is sensitive to the type of viscous operator used.

Physical diffusion operators hinder ZVI at high Reynolds numbers.

Radiative cooling suppresses ZVI unless the Peclet number exceeds ~10^4.

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.