Deconfinement Confined to the Heaviest Neutron Stars: Strange Quark Matter from a Flavor-Dressed Contact Interaction

Abstract

We study cold, electrically neutral, beta-equilibrated quark matter with u, d, and s quarks described by a symmetry-preserving contact interaction whose parameters are fixed in vacuum. The vector repulsion is varied as a fraction 0 < eta < 3/2 of the scalar coupling, and results are given with and without the flavor dressing of the effective coupling. Pure quark matter is not self-bound in this model, and no value of eta produces a star that reaches two solar masses while satisfying the tidal constraint from GW170817; the heaviest configurations owe their mass to an envelope of constituent quarks. We then construct hybrid stars with a relativistic mean-field hadronic phase fitted to GM1 saturation properties, using both Maxwell and Gibbs constructions and a deconfinement bag shift. Tying deconfinement to chiral restoration and requiring a continuous pressure yields an upper bound on this shift. Within that bound, quark matter appears inside stable stars only for eta < 1/2, never below about 4 n0, and only in stars between 2.2 and 2.35Msun, over a stable mass window of at most 0.02Msun. The vector strength that pure quark stars need to become massive prevents deconfinement in hybrid stars. Radii and tidal deformabilities of intermediate-mass stars are therefore set by the hadronic EoS alone; with the softer SLy4 EoS the transition moves to the maximum-mass configuration itself.