Modelling Neutron-Star Ocean Dynamics

TL;DR

This paper revisits neutron-star ocean mode calculations, critically examines the traditional approximation used in geophysics, and explores how rotation and relativistic effects influence mode behavior.

Contribution

It provides a detailed analysis of the traditional approximation's validity for neutron stars and introduces a method to incorporate relativistic effects into mode modeling.

Findings

Traditional approximation may be inaccurate for rapidly rotating neutron stars.

Ocean modes can transition from r-modes to gravity waves under certain conditions.

A prescription is proposed to extend Newtonian results to relativistic regimes.

Abstract

We re-visit the calculation of mode oscillations in the ocean of a rotating neutron star, which may be excited during thermonuclear X-ray bursts. Our present theoretical understanding of ocean modes relies heavily on the traditional approximation, commonly employed in geophysics. The approximation elegantly decouples the radial and angular sectors of the perturbation problem by neglecting the vertical contribution from the Coriolis force. However, as the implicit assumptions underlying it are not as well understood as they ought to be, we examine the traditional approximation and discuss the associated mode solutions. The results demonstrate that, while the approximation may be appropriate in certain contexts, it may not be accurate for rapidly rotating neutron stars. In addition, using the shallow-water approximation, we show analytically how the solutions that resemble r-modes change their nature in neutron-star oceans to behave like gravity waves. We also outline a simple prescription for lifting Newtonian results in a shallow ocean to general relativity, making the result more realistic.