2025ApJ...980..231L

In this work, under the hypothesis that quark matter may not be strange, we adopt a modification of the coupling constant of the four-quark scalar interaction $G\to {G}_{1}+{G}_{2}\langle \bar{\psi }\psi \rangle $ in the two-flavor Nambu-Jona-Lasinio (NJL) model to study nonstrange hybrid stars, where G₁ and G₂ are two parameters constrained by using the lattice quantum chromodynamics simulation results at the critical temperature and zero chemical potential. The Maxwell construction is used to describe the first-order confinement-deconfinement phase transition in hybrid stars. With recent measurements on neutron star mass, radius, and tidal deformability, the hybrid equations of state are constrained. It is found that pure nonstrange quark matter cores can exist in hybrid stars, possessing 0.026-0.04 M_⊙. The maximum hybrid star mass in the framework of the modified NJL model is about 0.1 M_⊙ lighter than that in the conventional two-flavor NJL model. It is argued that the binary neutron stars in GW170817 should be hadron stars.