Nonlinear r-Modes in Neutron Stars: Instability of an unstable mode

TL;DR

This paper investigates the nonlinear evolution of large amplitude r-modes in neutron stars through numerical simulations, revealing a catastrophic decay process that limits the mode amplitude below previous estimates.

Contribution

It provides new insights into the nonlinear dynamics and stability limits of r-modes in neutron stars, challenging earlier simulation results.

Findings

Large amplitude r-modes undergo catastrophic decay

Energy leaks into other fluid modes during decay

Decay leads to differentially rotating star configuration

Abstract

We study the dynamical evolution of a large amplitude r-mode by numerical simulations. R-modes in neutron stars are unstable growing modes, driven by gravitational radiation reaction. In these simulations, r-modes of amplitude unity or above are destroyed by a catastrophic decay: A large amplitude r-mode gradually leaks energy into other fluid modes, which in turn act nonlinearly with the r-mode, leading to the onset of the rapid decay. As a result the r-mode suddenly breaks down into a differentially rotating configuration. The catastrophic decay does not appear to be related to shock waves at the star's surface. The limit it imposes on the r-mode amplitude is significantly smaller than that suggested by previous fully nonlinear numerical simulations.