Abstract
A class of hybrid compact star equations of state is investigated that joins by a Maxwell construction a low-density phase of hadronic matter, modeled by a relativistic meanfield approach with excluded nucleon volume, with a high-density phase of color superconducting two-flavor quark matter, described within a nonlocal covariant chiral quark model. We find the conditions on the vector meson coupling in the quark model under which a stable branch of hybrid compact stars occurs in the cases with and without diquark condensation. We show that these hybrid stars do not form a third family disconnected from the second family of ordinary neutron stars unless additional (de)confining effects are introduced with a density-dependent bag pressure. A suitably chosen density dependence of the vector meson coupling assures that at the same time the $2~$M$_\odot$ maximum mass constraint is fulfilled on the hybrid star branch. A twofold interpolation method is realized which implements both, the density dependence of a confining bag pressure at the onset of the hadron-to-quark matter transition as well as the stiffening of quark matter at higher densities by a density-dependent vector meson coupling. For three parametrizations of this class of hybrid equation of state the properties of corresponding compact star sequences are presented, including mass twins of neutron and hybrid stars at 2.00, 1.39 and 1.20 $M_\odot$, respectively. The sensitivity of the hybrid equation of state and the corresponding compact star sequences to variations of the interpolation parameters at the 10% level is investigated and it is found that the feature of third family solutions for compact stars is robust against such a variation. This advanced description of hybrid star matter allows to interpret GW170817 as a merger not only of two neutron stars but also of a neutron star with a hybrid star or of two hybrid stars.
Highlights
We have investigated the question of whether it is possible to obtain a third family of compact stars for an equation of state resulting from the Maxwell construction of a first-order phase transition between a relativistic meanfield EoS of nuclear matter (DD2F_p40) and a nonlocal chiral quark model EoS of color superconducting twoflavor quark matter
We have applied in this work an interpolation procedure to reconstruct the thermodynamic behavior of a class of hybrid compact star equations of state which supports the existence of a third family of compact stars [22]
To this end we had to generalize the nonlocal chiral quark model by addition of a confining, density-dependent bag pressure BðμÞ that vanishes at high densities but at low densities leads to a sufficient softening of the EoS, resulting in a large density jump at the deconfinement transition that triggers the occurrence of an unstable branch
Summary
In the context of the observation of pulsars with high masses of about 2 M⊙ by Demorest et al [1,2,3] and Antoniadis et al [4], the question of the possible. In the present work we develop the interpolation technique for nonlocal covariant form-factor approaches further in order to fulfill the above requirements on models of compact star matter which would produce third family solutions for hybrid stars with quark matter cores that at the same time obey the constraints on their maximum mass to exceed the present observational bound of 2.01 Æ 0.04 M⊙ [4] and on their compactness to be in accord with the bounds derived from the gravitational waves detected for the inspiral phase of the binary compact star merger GW170817 by the LIGO and Virgo Collaboration (LVC) [8] To this end we suggest here a twofold interpolation. Approach that captures the density dependence of both the confining effects (the vanishing of a baglike negative pressure of the nonperturbative QCD medium) and the stiffness (the increase of the vector meson coupling strength)
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