Elastic stars in general relativity: IV. Axial perturbations

Abstract

This is the fourth paper in a series that attempts to put forward a consistent framework for modelling solid regions in neutron stars. Here we turn our attention to axial perturbations of spherically symmetric spacetimes using a gauge invariant approach due to one of us. Using the formalism developed in the first paper in the series it turns out that the matter perturbations are neatly expressible in terms of a ‘metric’ tensor field depending only on the speeds of shear wave propagation along the principal directions in the solid. The results are applicable to a wide class of elastic materials and do not assume material isotropy nor quasi-Hookean behaviour. The perturbation equations are then specialized to a static background and are given by two coupled wave equations. Our formalism is thus slightly simpler than the previously existing results of Schumaker and Thorne (1983 Mon. Not. R. Astron. Soc. 203 457), where an additional initial value equation needs to be solved. The simplification is mainly due to the gauge invariance of our approach and also shows up in somewhat simpler boundary conditions. We also give a first-order formulation suitable for numerical integration of the quasi-normal mode problem of a neutron star. The relations between the gauge independent variables and the, in general, gauge-dependent perturbed metric and strain tensor are explicitly given.