Two branches of neutron stars - reconciling a 2M_sun pulsar and SN1987A

The analysis of SN1987A led Brown and Bethe (1995) to conclusion, that the maximum mass of cold neutron stars is low, M_max ~ 1.5M_sun. Such a low M_max, due to a kaon condensation in the stellar core, implies collapse of a too massive deleptonized protoneutron star into a black hole. This would naturally explain the lack of a neutron star in the SN1987A remnant. On the other hand, recent evaluation of mass of PSR J0751+1807 gives M_max > 2M_sun. This contradicts the original Bethe-Brown model, but can be reconciled within scenarios proposed in the present Letter. We consider two types of dense matter models with high-density softening, due to a transition from a non-strange N-phase of matter to a strangeness carrying phase S: kaon condensation and deconfinement of quarks. Two scenarios of neutron star formation in stellar core collapse are considered. In the first scenario, realized in sufficiently hot and dense supernova cores, nucleation of an S-phase is sufficiently rapid so as to form an S-phase core, and implying M_max = M^S_max =~ 1.5M_sun. In the second scenario, nucleation of the S-phase at neutron star birth is too slow to materialize, and the star becomes cold without forming an S-phase core. Then, stellar mass can increase via accretion, until central density \rho_crit is reached, and the S phase forms. This N branch of neutron stars ends at M=M_crit. We select several models of N-phase satifying the necessary condition M^N_max > 2M_sun and combine them with models of kaon condensation and quark deconfinement. For kaon condensation, we get M_crit =~ M^S_max =~ 1.5M_sun, which is ruled out by PSR J0751+1807. On the contrary, for the EOSs with quark deconfinement we get M_crit =~ M^N_max > 2M_sun, which reconciles SN1987A and PSR J0751+1807.