Radial and Non-Radial Oscillations of Inverted Hybrid Stars

TL;DR

This paper investigates the oscillation modes of inverted hybrid stars with a quark crust and hadronic core, revealing unique gravitational-wave signatures that can distinguish them from other compact stars.

Contribution

It introduces the concept of inverted hybrid stars and analyzes their oscillation modes, highlighting distinctive features useful for gravitational-wave detection.

Findings

The fundamental radial and non-radial modes are similar to those of conventional hybrid stars.

The first non-radial p mode behaves inversely, appearing at higher frequencies.

Gravitational-wave signatures in specific frequency ranges can identify inverted hybrid stars.

Abstract

We study the radial and non-radial oscillations of Cross stars (CrSs), i.e., stars with a quark matter crust and a hadronic matter core in an inverted order compared to conventional hybrid stars. We draw comparisons of their oscillation modes with those of neutron stars, quark stars, and conventional hybrid stars. We find that the stellar stability analysis from the fundamental mode of radial oscillations, and the $g$, $f$ modes of non-radial oscillations are quite similar to those of conventional hybrid stars. However, due to the inverted stellar structure, the first non-radial $p$ mode of CrSs behaves in an inverted way and sits in a higher frequency domain compared to that of conventional hybrid stars. These results provide a direct way to discriminate CrSs from other types of compact stars via gravitational-wave probes. Specifically, compact stars emitting $g$-mode gravitational waves within the $0.5$-$1$ kHz range should be CrSs or conventional hybrid stars rather than neutron stars or pure quark stars, and a further GW detection of the first $p$ mode above 8 kHz or an identification of a decreasing trend of frequencies versus star masses associated with it will help identify the compact object to be a CrS rather than a conventional hybrid star.