Gravitational Waves from the Dynamical Bar Instability in a Rapidly Rotating Star

TL;DR

This study uses numerical simulations to demonstrate that a rapidly rotating star can maintain a long-lived bar shape, producing detectable gravitational waves, with implications for neutron star observations.

Contribution

The paper provides the first long-term simulation showing a stable, long-lived bar shape in a rapidly rotating star, supporting the possibility of persistent gravitational wave signals.

Findings

The bar shape persists for over 10 rotation periods.

Gravitational waves from such stars are detectable within our galaxy.

The results are scalable to different stellar scenarios.

Abstract

A rapidly rotating, axisymmetric star can be dynamically unstable to an m=2 "bar" mode that transforms the star from a disk shape to an elongated bar. The fate of such a bar-shaped star is uncertain. Some previous numerical studies indicate that the bar is short lived, lasting for only a few bar-rotation periods, while other studies suggest that the bar is relatively long lived. This paper contains the results of a numerical simulation of a rapidly rotating gamma=5/3 fluid star. The simulation shows that the bar shape is long lived: once the bar is established, the star retains this shape for more than 10 bar-rotation periods, through the end of the simulation. The results are consistent with the conjecture that a star will retain its bar shape indefinitely on a dynamical time scale, as long as its rotation rate exceeds the threshold for secular bar instability. The results are described in terms of a low density neutron star, but can be scaled to represent, for example, a burned-out stellar core that is prevented from complete collapse by centrifugal forces. Estimates for the gravitational-wave signal indicate that a dynamically unstable neutron star in our galaxy can be detected easily by the first generation of ground based gravitational-wave detectors. The signal for an unstable neutron star in the Virgo cluster might be seen by the planned advanced detectors. The Newtonian/quadrupole approximation is used throughout this work.