QCD color superconductivity in compact stars: Color-flavor locked quark star candidate for the gravitational-wave signal GW190814

At sufficiently high densities and low temperatures matter is expected to behave as a degenerate Fermi gas of quarks forming Cooper pairs, namely a color superconductor, as was originally suggested by Alford, Rajagopal, and Wilczek [Nucl. Phys. B537, 443 (1999)NUPBBO0550-321310.1016/S0550-3213(98)00668-3. The ground state is a superfluid, an electromagnetic insulator that breaks chiral symmetry, called the color-flavor locked phase. If such a phase occurs in the cores of compact stars, the maximum mass may exceed that of hadronic matter. The gravitational-wave signal GW190814 involves a compact object with mass 2.6 M Δ, within the so-called low mass gap. Since it is too heavy to be a neutron star and too light to be a black hole, its nature has not been identified with certainty yet. Here, we show not only that a color-flavor locked quark star with this mass is viable, but also we calculate the range of the model parameters, namely the superconducting gap Δ and the bag constant B, that satisfies the strict LIGO constraints on the equation of state. We find that a color-flavor locked quark star with mass 2.6 M Δ satisfies the observational constraints on the equation of state if Δ≥200 MeV and B≥83 MeV/fm3 for a strange quark mass ms=95 MeV/c2, and attains a radius (12.7-13.6) km and central density (7.5-9.8)1014 g/cm3.