The r-modes of slowly rotating, stratified neutron stars

TL;DR

This paper investigates how stratification in neutron stars affects the existence and nature of r-modes, revealing that stratification causes most overtones to merge with inertial modes, leaving only fundamental r-modes in certain conditions.

Contribution

It develops a framework to analyze r-modes in stratified neutron stars and shows how stratification alters mode characteristics, especially near the barotropic limit.

Findings

Most r-mode overtones merge with inertial modes in stratified stars.

Fundamental r-modes remain distinct in certain stratified models.

Mode behavior approaches inertial modes as the star nears barotropicity.

Abstract

The only r-modes that exist in a globally barotropic, rotating, Newtonian star are the fundamental $l = |m|$ solutions, where $l$ and $m$ are the indices of the spherical harmonic $Y_l^m$ that describe the mode's angular dependence. This is in stark contrast to a stellar model that is non-barotropic throughout its interior, which hosts all the $l \geq |m|$ perturbations including radial overtones. In reality, neutron stars are stratified with locally barotropic regions. Therefore, we explore how stratification alters a star's ability to support r-modes. We consider the globally stratified case and examine the behaviour of the modes as the star gets close to barotropicity. In this limit, we find that all but the fundamental $l = |m|$ perturbations change character and become generic inertial modes. Restricting the analysis to $l = |m|$ perturbations, we develop the r-mode equations in order to consider stellar models that exhibit local barotropicity. Our results for such models show that the r-mode overtones diverge and join the inertial modes. In order to see which r-modes persist and retain their character in realistic neutron stars, these calculations will need to be brought into full general relativity.