The subcritical baroclinic instability in local accretion disc models

TL;DR

This paper demonstrates the existence of a subcritical baroclinic instability in local accretion disc models through numerical simulations and analytical modeling, revealing its nonlinear nature and potential to transport angular momentum.

Contribution

It provides the first detailed numerical and analytical study of the subcritical baroclinic instability in local shearing box models, including its 3D behavior and physical mechanisms.

Findings

The instability occurs in flows stable by Solberg-Hoiland criterion.

It requires a radial entropy gradient and high thermal diffusivity.

The instability produces density waves transporting angular momentum outward.

Abstract

(abridged) Aims: We present new results exhibiting a subcritical baroclinic instability (SBI) in local shearing box models. We describe the 2D and 3D behaviour of this instability using numerical simulations and we present a simple analytical model describing the underlying physical process. Results: A subcritical baroclinic instability is observed in flows stable for the Solberg-Hoiland criterion using local simulations. This instability is found to be a nonlinear (or subcritical) instability, which cannot be described by ordinary linear approaches. It requires a radial entropy gradient weakly unstable for the Schwartzchild criterion and a strong thermal diffusivity (or equivalently a short cooling time). In compressible simulations, the instability produces density waves which transport angular momentum outward with typically alpha<3e-3, the exact value depending on the background temperature profile. Finally, the instability survives in 3D, vortex cores becoming turbulent due to parametric instabilities. Conclusions: The subcritical baroclinic instability is a robust phenomenon, which can be captured using local simulations. The instability survives in 3D thanks to a balance between the 2D SBI and 3D parametric instabilities. Finally, this instability can lead to a weak outward transport of angular momentum, due to the generation of density waves by the vortices.