Hybrid stars from a constrained equation of state

We determine, within a meta-model, the properties of the nuclear matter equation of state (EoS) that allow for a phase transition to deconfinement matter. It is shown that the properties that define the isoscalar channel are the ones that are affected, in particular, a phase transition implies much larger values of the skewness and kurtosis. The effect of multi-quark interaction channels in the description of the quark phase in hybrid stars is also studied. NS properties, such as the mass and radius of the quark core, show an interplay dependence between the 8-quark vector and the 4-quark isovector-vector interactions. We show that low mass NS, M ~ 1.4M⊙, may already contain a quark core, and satisfy all existing NS observational constraints. We discuss the strangeness content of the quark core and its influence on the speed of sound.

Thermodynamic Hadron-Quark Phase Transition of Chiral Nuclear Matter at High Temperature

Based on the extended Nambu-Jona–Lasinio (NJL) model with the scalar-vector eightpoint interaction [15], we consider what ultimately happens to exact chiral nuclear matter as it is heated. In the realm of very high temperature the fundamental degrees of freedom of the strong interaction, quarks and gluons, come into play and a transition from nuclear matter consisting of confined baryons and mesons to a state with ‘liberated’ quarks and gluons is expected. In this paper, the hadron-quark phase transition occurs above a limited temperature and after the chiral phase transition in the nuclear matter. There is a so-called quarkyonic- like phase, in which the chiral symmetry is restored but the elementary excitation modes are nucleonic at high density, appears just before deconfinement.PACS: 21.65.-f, 21.65.Mn, 11.30.Rd, 12.39.Ba, 25.75.Nq, 68.35.Rh

Hadron–Quark Phase Transition in the SU (3) Local Nambu–Jona-Lasinio (NJL) Model with Vector Interaction

We study the hadron–quark hybrid equation of state (EOS) of compact-star matter. The Nambu–Jona-Lasinio (NJL) local SU (3) model with vector-type interaction is used to describe the quark matter phase, while the relativistic mean field (RMF) theory with the scalar-isovector δ-meson effective field is adopted to describe the hadronic matter phase. It is shown that the larger the vector coupling constant GV, the lower the threshold density for the appearance of strange quarks. For a sufficiently small value of the vector coupling constant, the functions of the mass dependence on the baryonic chemical potential have regions of ambiguity that lead to a phase transition in nonstrange quark matter with an abrupt change in the baryon number density. We show that within the framework of the NJL model, the hypothesis on the absolute stability of strange quark matter is not realized. In order to describe the phase transition from hadronic matter to quark matter, Maxwell’s construction is applied. It is shown that the greater the vector coupling, the greater the stiffness of the EOS for quark matter and the phase transition pressure. Our results indicate that the infinitesimal core of the quark phase, formed in the center of the neutron star, is stable.

Twin stars: probe of phase transition from hadronic to quark matter

In agreement with the gravitational-wave events which are constantly increasing, new aspects of the internal structure of compact stars have come to light. A scenario in which a first order transition takes place inside these stars is of particular interest as it can lead, under conditions, to a third gravitationally stable branch (besides white dwarfs and neutron stars). This is known as the twin star scenario. The new branch yields stars with the same mass as normal compact stars but quite different radii. In the current work, we focus on hybrid stars undergone a hadron to quark phase transition near their core and how this new stable configuration arises. Emphasis is to be given especially in the aspects of the phase transition and its parametrization in two different ways, namely with Maxwell construction and with Gibbs construction. Qualitative findings of mass-radius relations of these stars will also be presented.

Thermodynamical phases in a PNJL model at zero temperature

The confinement/deconfinement transition described the Polyakov–Nambu–Jona–Lasinio (PNJL) model is extended to be operative at zero temperature regime. In this study, the scalar and vector channel interaction strengths of the original PNJL model are modified by introducing a dependence on the traced Polyakov loop. In such a way the effective interactions depend on the quark phase and in turn provides a backreaction of the quarks to the gluonic sector, also at zero temperature. On general grounds from quantum chromodynamics this is an expected feature. The thermodynamics of the extended model (PNJL0) is studied in detail. It presents along with a suitable choice of the Polyakov potential, a first order confined/deconfined quark phase transition even at $$T=0$$ T = 0 . We also show that the vector channel plays an important role in order to allow $$\varPhi \ne 0$$ Φ ≠ 0 solutions for the PNJL0 model. Furthermore, the sensitivity of the combined quarkyonic and deconfinement phases to the vector interaction strength and the proposed parametrization of the Polyakov-loop potential at $$T=0$$ T = 0 allowed to set a window for the bulk values of the relevant parameters.

Impact of quark deconfinement in neutron star mergers and hybrid star mergers

We describe an unambiguous gravitational-wave signature to identify the occurrence of a strong phase transition from hadronic matter to deconfined quark matter in neutron star mergers. Such a phase transition leads to a strong softening of the equation of state and hence to more compact merger remnants compared to purely hadronic models. If a phase transition takes place during merging, this results in a characteristic increase of the dominant postmerger gravitational-wave frequency relative to the tidal deformability characterizing the inspiral phase. By comparing results from different purely hadronic and hybrid models we show that a strong phase transition can be identified from a single, simultaneous measurement of pre- and postmerger gravitational waves. Furthermore, we present new results for hybrid star mergers, which contain quark matter already during the inspiral stage. Also for these systems we find that the postmerger GW frequency is increased compared to purely hadronic models. We thus conclude that also hybrid star mergers with an onset of the hadron-quark phase transition at relatively low densities may lead to the very same characteristic signature of quark deconfinement in the postmerger GW signal as systems undergoing the phase transition during merging.