Oscillations of Rapidly Rotating Superfluid Stars

TL;DR

This paper investigates non-axisymmetric oscillations of rapidly rotating superfluid neutron stars using linearized equations, revealing how entrainment and symmetry energy influence mode frequencies in fast rotation regimes.

Contribution

First detailed analysis of superfluid neutron star oscillations in the fast rotation regime considering entrainment and symmetry energy effects.

Findings

Entrainment and symmetry energy significantly affect mode splitting.

Symmetry energy alters inertial mode frequencies in rapid rotation.

Rotational effects are studied up to the mass-shedding limit.

Abstract

Using time evolutions of the relevant linearised equations we study non-axisymmetric oscillations of rapidly rotating and superfluid neutron stars. We consider perturbations of Newtonian axisymmetric background configurations and account for the presence of superfluid components via the standard two-fluid model. Within the Cowling approximation, we are able to carry out evolutions for uniformly rotating stars up to the mass-shedding limit. This leads to the first detailed analysis of superfluid neutron star oscillations in the fast rotation regime, where the star is significantly deformed by the centrifugal force. For simplicity, we focus on background models where the two fluids (superfluid neutrons and protons) co-rotate, are in beta-equilibrium and coexist throughout the volume of the star. We construct sequences of rotating stars for two analytical model equations of state. These models represent relatively simple generalisations of single fluid, polytropic stars. We study the effects of entrainment, rotation and symmetry energy on non-radial oscillations of these models. Our results show that entrainment and symmetry energy can have a significant effect on the rotational splitting of non-axisymmetric modes. In particular, the symmetry energy modifies the inertial mode frequencies considerably in the regime of fast rotation.