Dynamical bar-mode instability in rotating and magnetized relativistic stars

TL;DR

This study uses 3D simulations to explore how magnetic fields influence the dynamical bar-mode instability in rotating relativistic stars, revealing that strong magnetic fields can suppress the instability, affecting gravitational wave emission.

Contribution

It provides the first detailed analysis of magnetic field effects on bar-mode instability in relativistic stars using full general relativity simulations.

Findings

Weak magnetic fields have negligible impact on instability development.

Strong magnetic fields can completely suppress the bar-mode instability.

Magnetic field amplification occurs due to differential rotation.

Abstract

We present three-dimensional simulations of the dynamical bar-mode instability in magnetized and differentially rotating stars in full general relativity. Our focus is on the effects that magnetic fields have on the dynamics and the onset of the instability. In particular, we perform ideal-magnetohydrodynamics simulations of neutron stars that are known to be either stable or unstable against the purely hydrodynamical instability, but to which a poloidal magnetic field in the range of $10^{14}$--$10^{16}$ G is superimposed initially. As expected, the differential rotation is responsible for the shearing of the poloidal field and the consequent linear growth in time of the toroidal magnetic field. The latter rapidly exceeds in strength the original poloidal one, leading to a magnetic-field amplification in the the stars. Weak initial magnetic fields, i.e. $ \lesssim 10^{15}$ G, have negligible effects on the development of the dynamical bar-mode instability, simply braking the stellar configuration via magnetic-field shearing, and over a timescale for which we derived a simple algebraic expression. On the other hand, strong magnetic fields, i.e. $\gtrsim 10^{16}$ G, can suppress the instability completely, with the precise threshold being dependent also on the amount of rotation. As a result, it is unlikely that very highly magnetized neutron stars can be considered as sources of gravitational waves via the dynamical bar-mode instability.