Some properties of doubly-degenerate stars

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.