In this paper, we explore whether or not quark deconfinement may occur in high-mass neutron stars such as J1614-2230 (1.97 \pm 0.04 M_Sun) and J0348+0432 (2.01 \pm 0.04 M_Sun). Our study is based on a non-local extension of the SU(3) Nambu Jona-Lasinio (n3NJL) model with repulsive vector interactions among the quarks. This model goes beyond the frequently used local version of the Nambu Jona-Lasinio (NJL) model by accounting for several key features of QCD which are not part of the local model. Confined hadronic matter is treated in the framework of non-linear relativistic mean field theory. We find that both the local as well as the non-local NJL model predict the existence of extended regions of mixed quark-hadron (quark-hybrid) matter in high-mass neutron stars with masses of 2.1 to 2.4 M_Sun. Pure quark matter in the cores of neutron stars is obtained for certain parametrizations of the hadronic lagrangian and choices of the vector repulsion among quarks. The radii of high-mass neutron stars with quark-hybrid matter and/or pure quark matter cores in their centers are found to lie in the canonical range of 12 to 13 km.Comment: 31 pages, 17 figures, PRC accepted versio

Using a nonlocal extension of the SU(3) Nambu-Jona Lasinio model, which reproduces several of the key features of Quantum Chromodynamics, we show that mixed phases of deconfined quarks and confined hadrons (quark-hybrid matter) may exist in the cores of neutron stars as massive as around 2.1 M⊙. The radii of these objects are found to be in the canonical range of ∼ 12 − 13 km. According to our study, the transition to pure quark matter does not occur in stable neutron stars, but is shifted to neutron stars which are unstable against radial oscillations. The implications of our study for the recently discovered, massive neutron star PSR J1614-2230, whose gravitational mass is 1.97 ± 0.04 M⊙, are that this neutron star may contain an extended region of quark-hybrid matter at it center, but no pure quark matter. Introduction -Quantum Chromodynamics (QCD) has the properties of asymptotic freedom and confinement. The former implies that in the high-momentum transfer regime, the quarks behave essentially as free particles, i.e., the interaction between two quarks due to gluon exchange is very weak. This regime can therefore be treated using perturbation theory, where the quark-gluon coupling constant serves as an expansion parameter. For this momentum range, the dispersion processes can be calculated very accurately. By contrast, at low-momentum transfers ( 1 GeV) QCD becomes highly nonlinear, which prevents the use of perturbative methods. One of the renowned effective models that serves as a suitable approximation to QCD in the low-energy regime is the quark version of the Nambu JonaLasinio (NJL) model [1][2][3]. In this model chiral symmetry constraints are taken into account via effective interactions between quarks, through local four-point vertex interactions. The drawbacks of using local interactions are that the model must be regularized to avoid divergences in the loop integrals, and that the model is non-confining. The absence of confinement is essentially related to the fact that the dynamically generated constituent quark masses are momentum independent. Since the 1990's, there have been investigations proposing nonlocal interactions to solve these problems [4]. One interesting suggestion arises from the relationship between the NJL model and the model of one-gluon exchange where an effective gluon propagator is used to generate effective interactions between quarks. This provides a natural way to introduce a nonlocality in the quark-quark interaction, which can be characterized by a model-dependent form factor, g(p) [5].In this paper we analyze the global structure and composition of massive neutron stars in the framework of an extended version of the nonlocal SU(3) NJL model. Of particular interest is the question as to whether or not massive neutron stars, such as the recently discovered pulsar PSR J1614-2230 whose gravitational mass was found to be 1.97 ± 0.04 M ⊙ [6], may contain stellar cores made of deconfined quark matter [7][8][9]16]. Tendentially, one could argue that quark deconfinement may not...

High massive compact stars have been reported recently in the literature, providing strong constraints on the properties of the ultradense matter beyond the saturation nuclear density. In view of these results, the calculations of quark star or hybrid star equilibrium structure must be compatible with the provided observational data. But, since the used equations of state describing quark matter are in general too soft, in comparison with the equation of states used to describe the hadronic or nuclear matter, the calculated quark star models presented in the literature are in general not suitable to explain the stability of high compact massive objects.In this work, we present the calculations of spherically symmetric quark star structure by using an equation of state which takes into account the superconducting Color-Flavor Locked (CFL) phase of the strange quark matter. In addition, some fundamental aspects of QCD (asymptotic freedom and confinement) are considered by means of a phenomenological description of the deconfined quark phase, the density-dependent quark mass model (Chakrabarty et al 1989;Chakrabarty 1991). The quark matter behavior introduced by these model stiffens the corresponding equation of state. We thus investigate the influence of these models on the mass-radius diagram of quark stars. We obtain massive quark stars due to the stiffness of the equation of state, when a reasonable parametrization of the color superconducting gap is used. Models of quark stars enveloped by a nucleonic crust composed of a nuclear lattice embedded in an electron gas, with nuclei close to neutron drip line, are also discussed.Subject headings: equation of state: stars: neutron -X-rays: individual (EXO 0748-676, EXO 1745EXO -248, 4U 1608

Effects of excluded volume of nucleons on nuclear matter are studied, and the nuclear properties that follow from different relativistic mean-field model parametrizations are compared. We show that, for all tested parametrizations, the resulting volume energy a1 and the symmetry energy J are around the acceptable values of 16 MeV and 30 MeV, and the density symmetry L is around 100 MeV. On the other hand, models that consider only linear terms lead to incompressibility K0 much higher than expected. For most parameter sets there exists a critical point (ρc,δc), where the minimum and the maximum of the equation of state are coincident and the incompressibility equals zero. This critical point depends on the excluded volume parameter r. If this parameter is larger than 0.5 fm, there is no critical point and the pure neutron matter is predicted to be bound. The maximum value for neutron star mass is 1.85M⊙, which is in agreement with the mass of the heaviest observed neutron star 4U0900-40 and corresponds to r = 0.72 fm. We also show that the light neutron star mass (1.2M⊙) is obtained for r≃0.9 fm.

Abstract. This paper gives an brief overview of the structure of hypothetical strange quarks stars (quark stars, for short), which are made of absolutely stable 3-flavor strange quark matter. Such objects can be either bare or enveloped in thin nuclear crusts, which consist of heavy ions immersed in an electron gas. In contrast to neutron stars, the structure of quark stars is determined by two (rather than one) parameters, the central star density and the density at the base of the crust. If bare, quark stars possess ultra-high electric fields on the order of 10 18 to 10 19 V/cm. These features render the properties of quark stars more multifaceted than those of neutron stars and may allow one to observationally distinguish quark stars from neutron stars.