Oscillation dynamics of scalarized neutron stars

TL;DR

This paper investigates the oscillation behavior of scalarized neutron stars within scalar-tensor theories, analyzing how scalar fields influence mode frequencies and evaluating approximation methods for modeling these oscillations.

Contribution

It presents the first time evolution equations for perturbations of scalarized neutron stars including dynamic spacetime and scalar fields, and assesses the accuracy of Cowling approximations in this context.

Findings

Scalar field presence alters the relation between f-mode frequency and star density.

Cowling approximation can provide qualitatively correct and sometimes quantitatively accurate oscillation frequencies.

The study demonstrates the impact of scalar field parameters on neutron star oscillation modes.

Abstract

Scalar-tensor theories are well studied extensions of general relativity that offer deviations which are yet within observational boundaries. We present the time evolution equations governing the perturbations of a nonrotating scalarized neutron star, including a dynamic spacetime as well as scalar field within the framework of such scalar-tensor theories. We employ a theory that allows for a massive scalar field or a self-interaction term and we study the impact of those parameters on the non-axisymmetric $f$-mode. The time evolution approach allows for a comparatively simple implementation of the boundary conditions. We find that the $f$-mode frequency is no longer a simple function of the star's average density when a scalar field is present. We also evaluate the accuracy of different variants of the Cowling approximation commonly used in previous studies of neutron star oscillation modes in alternative theories of gravity and demonstrate that it can give us not only qualitatively correct results, but in some cases also good quantitative estimates of the oscillations frequencies.