Rapidly rotating axisymmetric neutron stars with quark cores

Abstract

We present a systematic study of the properties of pure hadronic and hybrid compact stars. The nuclear equation of state (EoS) for β-equilibrated neutron star matter was obtained using density dependent effective nucleon–nucleon interaction which satisfies the constraints from the observed flow data from heavy-ion collisions. The energy density of quark matter is lower than that of this nuclear EoS at higher densities implying the possibility of transition to quark matter inside the core. We solve the Einstein’s equations for rotating stars using pure nuclear matter and quark core. The β-equilibrated neutron star matter with a thin crust is able to describe highly massive compact stars but find that the nuclear to quark matter deconfinement transition inside neutron stars causes reduction in their masses. Recent observations of the binary millisecond pulsar J1614–2230 by Demorest et al. [1] suggest that the masses lie within 1.97±0.04M⊙ where M⊙ is the solar mass. In conformity with recent observations, pure nucleonic EoS determines that the maximum mass of NS rotating with frequency below r-mode instability is ∼1.95M⊙ with radius ∼10km. Although compact stars with quark cores rotating with Kepler’s frequency have masses up to ∼2M⊙, but if the maximum frequency is limited by the r-mode instability, the maximum mass ∼1.7 M⊙ turns out to be lower than the observed mass of 1.97±0.04M⊙, by far the highest yet measured with such certainty, implying exclusion of quark cores for such massive pulsars.