Color superconducting quark matter core in the third family of compact stars

Abstract

We investigate first order phase transitions from \ensuremath{\beta}-equilibrated hadronic matter to color-flavor-locked quark matter in a compact star interior. The hadronic phase including hyperons and a Bose-Einstein condensate of ${K}^{\ensuremath{-}}$ mesons is described by a relativistic field theoretical model with density dependent meson-baryon couplings. The early appearance of hyperons and/or a Bose-Einstein condensate of ${K}^{\ensuremath{-}}$ mesons delays the onset of the phase transition to higher density. In the presence of hyperons and/or a ${K}^{\ensuremath{-}}$ condensate, the overall equations of state become softer, resulting in smaller maximum masses than the cases without hyperons and a ${K}^{\ensuremath{-}}$ condensate. We find that the maximum mass neutron stars may contain a mixed phase core of hyperons, ${K}^{\ensuremath{-}}$ condensate, and color superconducting quark matter. Depending on the parameter space, we also observe that there is a stable branch of superdense stars called the third family branch beyond the neutron star branch. Compact stars in the third family branch may contain a pure color superconducting core and have radii smaller than those of the neutron star branch. Our results are compared with the recent observations on RX J185635-3754 and the recently measured mass-radius relationship by the X-ray Multi Mirror--Newton observatory.