Exotic Compact Objects with Two Dark Matter Fluids

Abstract

The generic properties of compact objects made of two different fluids of dark matter are studied in a scale-invariant approach. We investigate compact objects with a core–shell structure, where the two fluids are separated, and with mixed dark matter components, where both dark matter fluids are immersed within each other. The constellations considered are combinations of incompressible fluids, free and interacting Fermi gases, and equations of state with a vacuum term, i.e., self-bound dark matter. We find novel features in the mass–radius relations for combined dark matter compact objects, which distinguishes them from compact objects with a single dark matter fluid and compact stars made of ordinary baryonic matter, such as white dwarfs, neutron stars, and quark stars. The maximum compactness of certain combined dark matter stars can reach values up to the causality limit for compact stars but not beyond that limit if causality of the dark matter fluids is ensured.