Eigenmode frequency distribution of rapidly rotating neutron stars

TL;DR

This paper computes the oscillation mode frequencies of rapidly rotating neutron stars using perturbation theory and the Cowling approximation, revealing infinite spectra of pressure and inertial modes and their stability properties.

Contribution

It introduces a numerical code for calculating arbitrary order modes of rotating relativistic stars and analyzes the distribution and stability of these modes.

Findings

Infinite pressure mode spectrum extending to infinite frequency.

Existence of inertial modes within a finite frequency range depending on star properties.

Non-axisymmetric modes become unstable due to frequency shifts.

Abstract

We use perturbation theory and the relativistic Cowling approximation to numerically compute characteristic oscillation modes of rapidly rotating relativistic stars which consist of a perfect fluid obeying a polytropic equation of state. We present a code that allows the computation of modes of arbitrary order. We focus here on the overall distribution of frequencies. As expected, we find an infinite pressure mode spectrum extending to infinite frequency. In addition we obtain an infinite number of inertial mode solutions confined to a finite, well-defined frequency range which depends on the compactness and the rotation frequency of the star. For non-axisymmetric modes we observe how this range is shifted with respect to the axisymmetric ones, moving towards negative frequencies and thus making all m>2 modes unstable. We discuss whether our results indicate that the star's spectrum must have a continuous part, as opposed to simply containing an infinite number of discrete modes.