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 [Nuclear Physics B 537, 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 $2.6{\rm M}_\odot$, 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 $\Delta$ 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 $2.6{\rm M}_\odot$ satisfies the observational constraints on the equation of state if $\Delta \geq 200{\rm MeV}$ and $B\geq 83{ \rm MeV}/{\rm fm^3}$ for a strange quark mass $m_s=95~{\rm MeV}/c^2$, and attains a radius $(12.7-13.6) {\rm km}$ and central density $(7.5-9.8) 10^{14}{\rm g}/{\rm cm}^3$.