Quark-hadron phase transitions in Young and old neutron stars

The mixed phase of quarks and hadrons which might exist in the dense matter encountered in the varying conditions of temperature and trapped neutrino fraction in proto-neutron stars is studied. We show that hadronic equations of state that maximize the quark content of matter at a given density generally minimize the extent of the mixed phase region in a neutron star of a given mass, and that only in extreme cases could a pure quark star result. Neutrino trapping inhibits the appearance of a mixed phase which leads to possible proto-neutron star metastability. We also demonstrate that the temperature along adiabats in the quark-hadron mixed phase is much smaller than what is found for the kaon condensate-hadron mixed phase. This could lead to core temperatures which are significantly lower in stars containing quarks than in those not containing quarks.