Some properties of doubly-degenerate stars

TL;DR

This paper studies the properties of doubly-degenerate stars composed of nuclear matter and fermionic dark matter, analyzing their critical masses, geodesics, and orbital stability, revealing how composition influences their structure and dynamics.

Contribution

It introduces a detailed analysis of doubly-degenerate stars with mixed matter composition, highlighting the impact of dark matter envelope dominance on critical masses and orbital properties.

Findings

Critical masses depend on envelope composition.

Balanced matter mixtures can lead to smaller star masses.

Circular orbits can exist within the dark matter envelope, forming a gap with the innermost stable orbit.

Abstract

We investigate critical masses of and circular geodesics around doubly-degenerate stars (DDSs) which are composed of cold nuclear matter as well as cold Fermionic dark matter (DM). We here consider asymmetric dark Fermion with self-interaction as a DM candidate. These stars have core-envelope structures and are categorized into baryon-enveloped and DM-enveloped, according to the composition of their envelope. It is seen that the baryon-enveloped and DM-enveloped classes have their own critical masses determined mainly by the dominant component in their envelope. For a typical parameter sets, we see that a balanced mixture of two species may lead to smaller masses than if the either of the species is dominant. We also show that for a highly DM-enveloped case circular orbits in the vacuum region terminates at the innermost stable circular orbit (ISCO) in vacuum, but circular orbits of smaller radius are possible in the DM-envelope forming a gap between the ISCO and the inner orbits.