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The properties of isospin-asymmetric cold nuclear matter are studied in terms of the relativistic mean-field theory in which, besides the fields of σ, ω, and ρ mesons, and the isovector, Lorentz-scalar field of the δ-meson is also taken into account. The properties of purely nucleonic np matter are studied as a function of the baryon density nB and the asymmetry parameter α, as well as the properties of electrically neutral β-equilibrium npe μ matter as a function of the baryon density nB. For different values of nB and a, such characteristics of np matter as the energy per baryon, the specific energy owing to isospin asymmetry, the effective proton and neutron masses, and the specific binding energy, are determined. It is shown that the energy owing to the asymmetry for a fixed value of α is a monotonically increasing function of the baryon density nB. For npem matter, the effective proton and neutron masses M p e f f , M n e f f , the specific binding energy Ebind, the symmetry energy Esym, the quantitative fraction of protons Yp = np/nB are studied, as well as the average meson fields σ ~ , ω ~ , δ ~ , and ρ ~ as functions of the baryon density nB. [ABSTRACT FROM AUTHOR]

The integral parameters of neutron stars are studied taking into account the hadron-quark phase transition, which leads to the formation of a core of quark matter in the central part of a star. The quark matter is described using the local Nambu-Jona-Lasinio (NJL) model. The thermodynamic characteristics of the hadron matter are calculated in the framework of an extended version of the relativistic mean field (RMF) model that includes the contribution of the scalar-isovector δ -meson effective field. A Maxwell construction is used to determine the parameters of the phase transition. It is shown that for the equation of state examined here, stable hybrid stars correspond to a narrow range of values for the central density ρc ∈ (1.71÷1.73]·1015 g/cm3. In our model hybrid stars lie on the same branch as neutron stars, so that a branch with a third family is not formed. The maximum mass of a stable hybrid star is found to be _ Mmax = 2.05M⨀. The configuration with the maximum mass has a quark core with mass Mcore ≈ 10-3 M⨀ and radius Rcore ≈ 0.6 km. [ABSTRACT FROM AUTHOR]

The Maxwell and Glendenning construction scenarios of deconfinement phase transition in neutron star matter are investigated. The hadronic phase is described within the relativistic mean-field (RMF) theory, if the scalar-isovector δ-meson field is also taken into account. The strange quark phase is described in the frame of MIT bag model, including the effect of perturbative one-gluon exchange interactions. The influence of the δ-meson field on the deconfinement phase transition boundary characteristics is discussed. [ABSTRACT FROM AUTHOR]

The equation of state of neutron star matter is examined in terms of the relativistic mean-field theory, including a scalar-isovector δ-meson effective field. The constants of the theory are determined numerically so that the empirically known characteristics of symmetric nuclear matter are reproduced at the saturation density. The thermodynamic characteristics of both asymmetric nucleonic matter and β-equilibrium hadron-electron npe-plasmas are studied. Assuming that the transition to strange quark matter is an ordinary first-order phase transition described by Maxwell's rule, a detailed study is made of the variations in the parameters of the phase transition owing to the presence of a δ-meson field. The quark phase is described using an improved version of the bag model, in which interactions between quarks are accounted for in a one-gluon exchange approximation. The characteristics of the phase transition are determined for various values of the bag parameter within the range B ∈ [60,120]MeV/fm3 and it is shown that including a δ-meson field leads to a reduction in the phase transition pressure P0 and in the concentrations n N and n Q at the phase transition point. [ABSTRACT FROM AUTHOR]

We consider an equation of state that leads to a first-order phase transition from the nucleon state to the quark state with a transition parameter λ > 3/2 (λ = ρ[sub Q]/(ρ[sub N] + P[sub 0]/c²)) in superdense nuclear matter. Our calculations of integrated parameters for superdense stars using this equation of state show that on the stable branch of the dependence of stellar mass on central pressure (dM/dP[sub c] > 0) in the range of low masses, a new local maximum with M[sub max] = 0.082[sub ⊙] and R = 1251 km appears after the formation of a toothlike kink (M = 0.08M[sub ⊙], R = 205 km) attributable to quark production. For such a star, the mass and radius of the quark core are M[sub core] = 0.005M[sub ⊙] and R[sub core] = 1.73 km, respectively. In the model under consideration, mass accretion can result in two successive transitions to a quark-core neutron star with energy release similar to a supernova explosion: initially, a low-mass star with a quark core is formed; the subsequent accretion leads to configurations with a radius of ∼ 1000 km; and, finally, the second catastrophic restructuring gives rise to a star with a radius of ∼ 100 km. [ABSTRACT FROM AUTHOR]