Formation of millisecond pulsars with CO white dwarf companions - I. PSR J1614-2230: evidence for a neutron star born massive

The recent discovery of a 2M binary millisecond pulsar not only has important consequences for the equation of state of nuclear matter at high densities but also raises the interesting question of whether the neutron star PSR J1614−2230 was born massive. The answer is vital for understanding neutron star formation in core collapse supernovae. Furthermore, this system raises interesting issues about the nature of the progenitor binary and how it evolved during its mass-exchanging X-ray phase. In this paper we discuss the progenitor evolution of PSR J1614−2230. We have performed detailed stellar evolution modelling of intermediatemass X-ray binaries undergoing Case A Roche lobe overflow (RLO) and applied an analytic parametrization for calculating the outcome of either a common envelope evolution or the highly super-Eddington isotropic re-emission mode. We find two viable possibilities for the formation of the PSR J1614−2230 system: either it contained a 2.2–2.6M giant donor star and evolved through a common envelope and spiral-in phase or, more likely, it descended from a close binary system with a 4.0–5.0M main-sequence donor star via Case A RLO.We conclude that the neutron star must have been born with a mass of either ∼1.95M or 1.7 ± 0.15M , which significantly exceeds neutron star birth masses in previously discovered radio pulsar systems. Based on the expected neutron star birth masses from considerations of stellar evolution and explosion models, we find it likely that the progenitor star of PSR J1614−2230 was more massive than 20M .