Hybrid Stars in the Framework of the Local Nambu - Jona-Lasinio Model for Quark Matter.

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 $\delta$-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. In our model hybrid stars lie on the same branch as neutron stars, so that a branch with a third family is not formed. 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 $\rho_c\in(1.71 \div 1.73]\cdot 10^{15}$ g/cm$^3$. 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 $M_{max} = 2.05 M_\odot$. The configuration with the maximum mass has a quark core with mass $M_{core} \approx 10^{-3} M_\odot$ and radius $R_{core} \approx 0.6$ km.