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

Abstract

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). 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 $2.6\text{ }\text{ }{\mathrm{M}}_{\ensuremath{\bigodot}}$, 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 $\mathrm{\ensuremath{\Delta}}$ 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 $2.6\text{ }\text{ }{\mathrm{M}}_{\ensuremath{\bigodot}}$ satisfies the observational constraints on the equation of state if $\mathrm{\ensuremath{\Delta}}\ensuremath{\ge}200\text{ }\text{ }\mathrm{MeV}$ and $B\ensuremath{\ge}83\text{ }\text{ }\mathrm{MeV}/{\mathrm{fm}}^{3}$ for a strange quark mass ${m}_{s}=95\text{ }\text{ }\mathrm{MeV}/{c}^{2}$, and attains a radius $(12.7--13.6)\text{ }\text{ }\mathrm{km}$ and central density $(7.5--9.8){10}^{14}\text{ }\text{ }\mathrm{g}/{\mathrm{cm}}^{3}$.