Hybrid stars among mass gap objects are excluded by twin stars at $1.4\,M_\odot$

Abstract

We test whether compact objects in the so called mass gap ($2.5 < M_{\rm max}/M_\odot < 5.0$) can be hybrid stars. Using a generic hybrid equation of state with a first-order deconfinement transition, we map the allowed parameter space in a Seidov-type diagram and confront it with modern mass--radius constraints. We find that mass-gap hybrid stars require an extremely early onset of deconfinement and very stiff quark matter. The Bayesian analysis, however, favors equations of state with deconfinement at typical neutron-star masses around $1.4\,M_\odot$ with mass-twin stars that, if confirmed, would rule out hybrid stars as candidates for observed mass-gap compact objects.