Inertial modes of slowly rotating relativistic stars in the Cowling approximation

Abstract

We study oscillations of slowly rotating relativistic barotropic as well as non-barotropic polytropic stars in the Cowling approximation, including first order rotational corrections. By taking into account the coupling between the polar and axial equations, we find that, in contrast to previous results, the $m=2$ $r$ modes are essentially unaffected by the continuous spectrum and exist even for very relativistic stellar models. We perform our calculations both in the time and frequency domain. In order to numerically solve the infinite system of coupled equations, we truncate it at some value $l_{\rm max}$. Although the time dependent equations can be numerically evolved without any problems, the eigenvalue equations possess a singular structure, which is related to the existence of a continuous spectrum. This prevents the numerical computation of an eigenmode if its eigenfrequency falls inside the continuous spectrum. The properties of the latter depend strongly on the cut-off value $l_{\rm max}$ and it can consist of several either disconnected or overlapping patches, which are the broader the more relativistic the stellar model is. By discussing the dependence of the continuous spectrum as a function of both the cut-off value $l_{\rm max}$ and the compactness $M/R$, we demonstrate how it affects the inertial modes. Through the time evolutions we are able to show that some of the inertial modes can actually exist inside the continuous spectrum, but some cannot. For more compact and therefore more relativistic stellar models, the width of the continuous spectrum strongly increases and as a consequence, some of the inertial modes, which exist in less relativistic stars, disappear.