The evolution and masses of the neutron star and donor star in the high mass X-ray binary OAO

We report near-infrared radial velocity measurements of the recently identified donor star in the high mass X-ray binary system OAO 1657–415 obtained in the H band using ISAAC on the VLT. Cross-correlation methods were employed to construct a radial velocity curve with a semi-amplitude of 22.1± 3.5 km s. Combined with other measured parameters of this system this provides a dynamically determined neutron star mass of 1.42±0.26M⊙ and a mass of 14.3± 0.8 M⊙ for the Ofpe/WN9 highly evolved donor star. OAO 1657–415 is an eclipsing High Mass X-ray binary pulsar with the largest eccentricity and orbital period of any within its class. Of the ten known eclipsing Xray binary pulsars OAO 1657–415 becomes the ninth with a dynamically determined neutron star mass solution and only the second in an eccentric system. Furthermore, the donor star in OAO 1657–415 is much more highly evolved than the majority of the supergiant donors in other High Mass X-ray binaries (HMXBs), joining a small but growing list of HMXBs donors with extensive hydrogen depleted atmospheres. Considering the evolutionary development of OAO 1657–415, we have estimated the binding energy of the envelope of the mass donor and find that there is insufficient energy for the removal of the donor’s envelope via spiral-in, ruling out a Common Envelope evolutionary scenario. With its non-zero eccentricity and relatively large orbital period the identification of a definitive evolutionary pathway for OAO 1657– 415 remains problematic, we conclude by proposing two scenarios which may account for OAO 1657–415 current orbital configuration.