Oscillation modes of hybrid stars within the relativistic Cowling approximation

TL;DR

This paper investigates oscillation modes of hybrid neutron stars using the relativistic Cowling approximation, highlighting how phase transitions and color superconductivity influence gravitational wave signals, especially $g$-modes.

Contribution

It provides new insights into how hadron-quark phase transitions and color superconductivity affect oscillation frequencies in hybrid stars.

Findings

$g$-mode frequencies relate to quark matter properties.

Color superconductivity increases oscillation frequencies.

$g$-modes between 1-1.5 kHz indicate phase transition presence.

Abstract

The first direct detection of gravitational waves has opened a new window to study the Universe and would probably start a new era: the gravitational wave Astronomy. Gravitational waves emitted by compact objects like neutron stars could provide significant information about their structure, composition and evolution. In this paper we calculate, using the relativistic Cowling approximation, the oscillations of compact stars focusing on hybrid stars, with and without a mixed phase in their cores. We study the existence of a possible hadron-quark phase transition in the central regions of neutron stars and the changes it produces on the gravitational modes frequencies emitted by these stars. We pay particular attention to the $g$-modes, which are extremely important as they could signal the existence of pure quark matter inside neutron stars. Our results show a relationship between the frequency of the $g$-modes and the constant speed of sound parametrization for the quark matter phase. We also show that the inclusion of color superconductivity produces an increase on the oscillation frequencies. We propose that observations of $g$-modes with frequencies $f_{\rm g}$ between $1$ kHz and $1.5$ kHz should be interpreted as an evidence of a sharp hadron-quark phase transition in the core of a compact object.