Parametric instability in warped astrophysical discs: growth, saturation and feedback

TL;DR

This paper investigates the parametric instability in warped astrophysical discs, analyzing its growth, saturation, and feedback effects through numerical simulations, and proposes a viscous alpha model to represent the turbulence.

Contribution

It introduces a novel numerical framework to study the nonlinear saturation of parametric instability in warped discs and develops an effective viscous model for the turbulence.

Findings

Identified several locally growing modes consistent with theoretical predictions.

Found that wave breaking suppresses short-wavelength modes, leaving a dominant long-wavelength turbulence.

Demonstrated that a time-dependent anisotropic viscous model accurately captures warp evolution.

Abstract

Attempts to understand the dynamics of warped astrophysical discs have garnered significant attention, largely motivated by the growing catalogue of observed distorted systems. Previous studies have shown that the evolution of the warp is crucially regulated by the internal flow fields established by the undulating geometry. These are typically modelled as laminar horizontal, shearing flows which oscillate back and forth at approximately the orbital frequency. However this shearing motion is known to be susceptible to a hydrodynamic, parametric instability of inertial waves which might modify the warped dynamics. Whilst the linear growth phase is well understood, the subsequent nonlinear saturation combined with the self-consistent feedback onto the warp has not been studied. In this work, we implement a novel numerical setup using the recent ring model framework of Fairbairn and Ogilvie, within the Lagrangian code GIZMO. We formally identify several locally growing modes in the simulation, as predicted by a three-mode coupling analysis of the instability, and find decent agreement with the theoretical growth rates. We understand the saturation mechanism as a wave breaking process which suppresses the growth of shorter wavelength parametric couplings first, whilst allowing the longest mode to dominate the final quasi-steady, wavelike turbulence. The Reynolds stresses, transporting energy from the warp to the small scales, can be effectively modelled using a time-dependent, anisotropic viscous alpha model which closely captures the amplitude and phase evolution of the warp. Consequently, this model might help inform future global studies which are commonplace but typically don't resolve the parametric instability.