Modelling neutron star mountains in relativity

TL;DR

This paper investigates the maximum size of neutron star mountains in relativistic gravity by modeling their deformation under different forces and equations of state, revealing sensitivity to the source mechanism and nuclear matter properties.

Contribution

It introduces a perturbative method to calculate neutron star mountains in relativity and explores their dependence on deformation sources and equations of state.

Findings

Largest mountains depend on the deformation mechanism.

Mountains are sensitive to the nuclear matter equation of state.

Crust yielding limits set the maximum mountain size.

Abstract

Rapidly spinning, deformed neutron stars have long been considered potential gravitational-wave emitters. However, so far only upper limits on the size of the involved quadrupole deformations have been obtained. For this reason, it is pertinent to ask how large a mountain can be before the neutron star crust fractures. This is the question we consider in this paper, which describes how mountains can be calculated in relativistic gravity. Formally, this is a perturbative calculation that requires a fiducial force to source the mountain. Therefore, we consider three simple examples and increase their deforming amplitudes until the crust yields. We demonstrate how the derived mountains depend on the equation of state by considering a range of models obtained from chiral effective field theory. We find that the largest mountains depend sensitively on both the mechanism that sources them and the nuclear-matter equation of state.