Hybrid stars within a covariant, nonlocal chiral quark model

We present a hybrid equation of state (EoS) for dense matter in which a nuclear matter phase is described within the Dirac-Brueckner-Hartree-Fock (DBHF) approach and a two-flavor quark matter phase is modelled according to a recently developed covariant, nonlocal chiral quark model. We show that modern observational constraints for compact star masses ($M~2M_{\odot }$) can be satisfied when a small vector-like four quark interaction is taken into account. The corresponding isospin symmetric EoS is consistent with flow data analyses of heavy ion collisions and points to a deconfinement transition at about 0.55 fm${}^{-3}$.

Figure 1

Pressure as a function of the baryochemical potential for isospin symmetric matter. The nuclear matter phase is modelled by the DBHF equation of state with the Bonn-A potential [33] (solid line) and the results for the covariant, nonlocal chiral quark model are given for different scaled vector coupling strengths $g=0.03,0.05,0.07,0.09$ (dotted, dash-dotted, dashed, and dash-double-dotted curves, respectively) and scaled diquark coupling strengths of $h=0.70$ (left panel) and $h=0.74$ (right panel). A phase transition to quark matter is obtained at the crossing of nuclear and quark matter curves, for discussion see text.

Figure 2

Pressure as a function of the density for isospin symmetric matter. The phase transition to quark matter softens the EoS at densities above $0.55$ fm${}^{-3}$, relative to the nuclear DBHF EoS, thus allowing us to fulfill the flow constraint derived in Ref. [47]. Line styles as in Fig. 1.

Figure 3

Mass-radius relationships for neutron star configurations (DBHF EoS, solid line) and hybrid star configurations with hadronic shell (DBHF EoS) and color superconducting quark matter core (nonlocal chiral quark model EoS) for two parameter sets characterized by the coupling ratios $h=H/G_S$ and $g=G_V/G_S$. Dashed line: $h=0.74,g=0.07$; dash-dotted line: $h=0.70,g=0.05$.