On the convective overstability in protoplanetary discs


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Authors
Latter, HN 
Abstract

This paper explores the driving of low-level hydrodynamical activity in protoplanetary-disc dead zones. A small adverse radial entropy gradient, ordinarily stabilized by rotation, excites oscillatory convection (‘convective overstability’) when thermal diffusion, or cooling, is neither too strong nor too weak. I revisit the linear theory of the instability, discuss its prevalence in protoplanetary discs, and show that unstable modes are exact non-linear solutions in the local Boussinesq limit. Overstable modes cannot grow indefinitely, however, as they are subject to a secondary parametric instability that limits their amplitudes to relatively low levels. If parasites set the saturation level of the ensuing turbulence then the convective overstability is probably too weak to drive significant angular momentum transport or to generate vortices. But I also discuss an alternative, and far more vigorous, saturation route that generates radial ‘layers’ or ‘zonal flows’ (witnessed in semiconvection). Numerical simulations are required to determine which outcome is favoured in realistic discs, and consequently how important the instability is for disc dynamics.

Description
Keywords
convection, instabilities, turbulence, waves, protoplanetary discs
Journal Title
Monthly Notices of the Royal Astronomical Society
Conference Name
Journal ISSN
0035-8711
1365-2966
Volume Title
455
Publisher
Oxford University Press (OUP)
Sponsorship
Science and Technology Facilities Council (ST/L000636/1)
This research is partially funded by STFC grant ST/L000636/1.