Hydrodynamic instability in warped astrophysical discs

TL;DR

This paper investigates the hydrodynamic instability in warped astrophysical discs, revealing that small warps can induce turbulence through parametric resonance of inertial waves, affecting disc evolution.

Contribution

It introduces a linear stability analysis of warped disc flows using a warped shearing sheet and identifies a parametric resonance mechanism causing instability.

Findings

Hydrodynamic instability arises from inertial wave resonance.

Small warps can generate turbulence in low-viscosity discs.

Warp-induced wave activity can alter large-scale disc dynamics.

Abstract

Warped astrophysical discs are usually treated as laminar viscous flows, which have anomalous properties when the disc is nearly Keplerian and the viscosity is small: fast horizontal shearing motions and large torques are generated, which cause the warp to evolve rapidly, in some cases at a rate that is inversely proportional to the viscosity. However, these flows are often subject to a linear hydrodynamic instability, which may produce small-scale turbulence and modify the large-scale dynamics of the disc. We use a warped shearing sheet to compute the oscillatory laminar flows in a warped disc and to analyse their linear stability by the Floquet method. We find widespread hydrodynamic instability deriving from the parametric resonance of inertial waves. Even very small, unobservable warps in nearly Keplerian discs of low viscosity can be expected to generate hydrodynamic turbulence, or at least wave activity, by this mechanism.