Modeling hybrid stars with an SU(3) nonlinear σ model

We study the behavior of hybrid stars by using an extended hadronic and quark SU(3) nonlinear sigma model. The degrees of freedom change naturally, in this model, from hadrons to quarks as the density/temperature increases. At zero temperature, we reproduce massive neutron stars, which contain cores of hybrid matter of 2 km for the nonrotating case and 1.18 and 0.87 km, in the equatorial and polar directions, respectively, for stars that rotate at the Kepler frequency (physical cases lie in between). The cooling of such stars is also analyzed.