HD 49798: Its History of Binary Interaction and Future Evolution

The bright subdwarf-O star (sdO) HD 49798 is in a 1.55 day orbit with a compact companion that is spinning at 13.2 s. Using the measurements of the effective temperature (T-eff), surface gravity (log g), and surface abundances of the sdO, we construct models to study the evolution of this binary system using Modules for Experiments in Stellar Astrophysics (MESA). Previous studies of the compact companion have disagreed on whether it is a white dwarf (WD) or a neutron star (NS). From the published measurements of the companion's spin and spin-up rate, we agree with Mereghetti and collaborators that an NS companion is more likely. However, since there remains the possibility of a WD companion, we use our constructed MESA models to run simulations with both WD and NS companions that help us constrain the past and future evolution of this system. If it presently contains an NS, the immediate mass transfer evolution upon Roche lobe filling will lead to mass transfer rates comparable to that implied in ultraluminous X-ray sources (ULXs). Depending on the rate of angular momentum extraction via a wind, the fate of this system is either a wide (P-orb approximate to 3 day) intermediate-mass binary pulsar (IMPB) with a relatively rapidly spinning NS (approximate to 0.3 s) and a high mass WD (approximate to 0.9 M-circle dot), or a solitary millisecond pulsar (MSP).