Focusing of nonlinear eccentric waves in astrophysical discs. II. Excitation and damping of tightly-wound waves

TL;DR

This paper develops a nonlinear theory for tightly-wound eccentric waves in astrophysical discs, incorporating viscosity, and confirms their excitation and damping behaviors through analysis and comparison with simulations.

Contribution

It extends previous linear theories to 3D discs with viscosity, revealing how shear viscosity excites and damps nonlinear eccentric waves.

Findings

Linear eccentric waves are overstable and excited by shear viscosity.

Weakly nonlinear waves remain excited, but damping occurs at higher nonlinearities.

Eccentric wave damping near black hole orbits leads to nearly circular discs.

Abstract

In this paper I develop a nonlinear theory of tightly-wound (highly twisted) eccentric waves in astrophysical discs, based on the averaged Lagrangian method of Whitham. Viscous dissipation is included in the theory by use of a pseudo-Lagrangian. This work is an extension of the theory developed by Lee \& Goodman to 3D discs, with the addition of viscosity. I confirm that linear tightly-wound eccentric waves are overstable and are excited by the presence of a shear viscosity and show this persists for weakly nonlinear waves. I find the waves are damped by shear viscosity when the wave become sufficiently nonlinear, a result previously found in particulate discs. Additionally I compare the results of this model to recent simulations of eccentric waves propagating in the inner regions of black hole discs and show that an ingoing eccentric wave can be strongly damped near the marginally stable orbit, resulting in a nearly circular disc with a strong azimuthal variation in the disc density.