Two-Fluid Radial Stability of Dark-Matter-Admixed Compact Objects: Dark Compact Planets, Dark White Dwarfs, Dark Neutron Stars, and Dark Oysters

Abstract

We investigate the equilibrium structure and stability against radial oscillations of a variety of two-fluid compact objects containing ordinary baryonic matter admixed with cold dark matter coupled through the background spacetime. For the ordinary fluid, we employ a QCD equation of state compatible with chiral effective field theory at nuclear densities (supplemented by the BPS and Negele-Vautherin models at extremely low densities) and with perturbative QCD in the high-density regime while also satisfying also current astrophysical data. For the dark fluid, we assume self-interacting fermionic matter with particle masses of $m_f=$ 2, 5, 10, and 100 GeV and two self-interaction strengths of $y=0.1$ and $10^3$. By scanning a large range of central pressures for both fluids, we identify the presence of dark compact planets, dark white dwarfs, dark neutron stars and the so-called dark oysters. Importantly, these objects are not only in two-fluid hydrostatic equilibrium, but they are also stable against two-fluid radial oscillations. We find that the relative representation of each class in the radially stable sample is strongly sensitive to the combinations of $m_f$ and $y$, with some combinations entirely suppressing some of these object classes within the sampled domain while favoring others. In addition to these classes, strong self-repulsion supports stable, dark-matter-dominated branches with compact ordinary-matter cores and extended dark halos, approaching the self-interacting fermion-star limit. Finally, we calculate the gravitational redshift at the ordinary-matter surface, $z_{\rm OM}$, including the contribution of extended dark-matter halos for which we found radially stable configurations with $z_{\rm OM}>2$.