Influence of dark matter on hybrid and twin stars

We investigate the impact of dark matter (DM) on hybrid and twin stars within a two-fluid framework, where DM and normal matter interact only through gravity. A self-interacting fermionic DM model is considered, while for the nuclear and quark matter we employ relativistic mean-field models and the constant sound-speed parameterization, respectively. Solving the two-fluid Tolman-Oppenheimer-Volkoff equations over a five-dimensional parameter space spanning the quark matter transition pressure $p_t$, energy density jump $\Delta\epsilon$, sound speed $C_s^2$, DM particle mass $m_\chi$, and DM fraction $f_\chi$, we classify the resulting configurations into hybrid stars and four twin star categories and map the onset properties of the quark phase. The effect of DM on the hybrid and twin star population is regime dependent: the DM particle mass determines whether DM forms an extended halo or a compact core, while the DM fraction sets the magnitude of the effect. In the halo regime, DM enhances quark matter formation and increases the number of hybrid and twin stars; in the core regime, it suppresses them. DM also stabilizes hybrid and twin configurations that are unstable in the pure-baryonic case and systematically pushes the nucleon-to-quark transition to higher densities, with the onset pressure, mass, and radius all rising with the DM fraction, potentially hiding quark matter from current neutron star observations.