On the vertical-shear instability in astrophysical discs

TL;DR

This paper investigates the linear stability of astrophysical discs with vertical shear, revealing that the fastest-growing modes are surface-localized and short-wavelength, which complicates numerical simulations and has implications for disc dynamics.

Contribution

The study extends previous models to include global axisymmetric perturbations and more realistic entropy profiles, providing new insights into the nature of the vertical-shear instability.

Findings

Fastest-growing modes are surface-localized with short wavelengths.

Global inertial (r-) modes can also be destabilized.

The instability is ill-posed, with growth on the shortest scales.

Abstract

We explore the linear stability of astrophysical discs exhibiting vertical shear, which arises when there is a radial variation in the temperature or entropy. Such discs are subject to a "vertical-shear instability", which recent nonlinear simulations have shown to drive hydrodynamic activity in the MRI-stable regions of protoplanetary discs. We first revisit locally isothermal discs using the quasi-global reduced model derived by Nelson et al. (2013). This analysis is then extended to global axisymmetric perturbations in a cylindrical domain. We also derive and study a reduced model describing discs with power law radial entropy profiles ("locally polytropic discs"), which are somewhat more realistic in that they possess physical (as opposed to numerical) surfaces. In all cases the fastest growing modes have very short wavelengths and are localised at the disc surfaces (if present), where the vertical shear is maximal. An additional class of modestly growing vertically global body modes is excited, corresponding to destabilised classical inertial waves ("r-modes"). We discuss the properties of both types of modes, and stress that those that grow fastest occur on the shortest available length scales (determined either by the numerical grid or the physical viscous length). This ill-posedness makes simulations of the instability difficult to interpret. We end with some brief speculation on the nonlinear saturation and resulting angular momentum transport.