  The recent discovery of a 2 M_sun binary millisecond pulsar (Demorest et al.
2010) has not only important consequences for the equation-of-state of nuclear
matter at high densities but also raises the interesting question if 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 intermediate-mass X-ray binaries undergoing Case
A Roche-lobe overflow (RLO) and applied an analytic parameterization 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 2.2-2.6
M_sun 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 4.0-5.0
M_sun main sequence donor star via Case A RLO. We conclude that the neutron
star must have been born with a mass of ~1.95 M_sun or 1.7+-0.15 M_sun,
respectively - 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 that the progenitor star of PSR J1614-2230 is likely to have been more
massive than 20 M_sun.

Solar and Stellar Astrophysics;High Energy Astrophysical Phenomena