Viscous damping of r-modes: Large amplitude saturation

TL;DR

This paper investigates how non-linear viscous effects influence the damping of r-mode oscillations in compact stars, revealing that large-amplitude damping can significantly suppress r-mode growth and impact stellar spin evolution.

Contribution

It provides the first detailed analysis of large-amplitude viscous damping in different types of compact stars, including analytical expressions and numerical results.

Findings

Large-amplitude damping times are much shorter than previously estimated.

Damping can counteract gravitational instability across relevant frequencies.

Viscous damping may prevent r-mode growth without additional hydrodynamic mechanisms.

Abstract

We analyze the viscous damping of r-mode oscillations of compact stars, taking into account non-linear viscous effects in the large-amplitude regime. The qualitatively different cases of hadronic stars, strange quark stars, and hybrid stars are studied. We calculate the viscous damping times of r-modes, obtaining numerical results and also general approximate analytic expressions that explicitly exhibit the dependence on the parameters that are relevant for a future spindown evolution calculation. The strongly enhanced damping of large amplitude oscillations leads to damping times that are considerably lower than those obtained when the amplitude dependence of the viscosity is neglected. Consequently, large-amplitude viscous damping competes with the gravitational instability at all physical frequencies and could stop the r-mode growth in case this is not done before by non-linear hydrodynamic mechanisms.