Hadron–quark crossover and massive hybrid stars

On the basis of the percolation picture from the hadronic phase with hyperons to the quark phase with strangeness, we construct a new equation of state (EOS) with the pressure interpolated as a function of the baryon density. The maximum mass of neutron stars can exceed 2M_⊙ if the following two conditions are satisfied: (1) the crossover from hadronic matter to quark matter takes place at around three times the normal nuclear matter density, and (2) the quark matter is strongly interacting in the crossover region and has a stiff equation of state. This is in contrast to the conventional approach, assuming the first-order phase transition in which the EOS always becomes soft due to the presence of the quark matter at high density. Although the choice of the hadronic EOS does not affect the above conclusion for the maximum mass, the three-body force among nucleons and hyperons plays an essential role in the onset of hyperon mixing and the cooling of neutron stars. (author)