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 seconds. Using the measurements of the effective temperature ($T_{\rm eff}$), 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 ($\texttt{MESA}$). 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 a NS companion is more likely. However, since there remains the possibility of a WD companion, we use our constructed $\texttt{MESA}$ 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 a NS, the immediate mass transfer evolution upon Roche lobe (RL) 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 ($P_{\rm orb}{\approx} 3$ day) intermediate mass binary pulsar (IMPB) with a relatively rapidly spinning NS (${\approx} 0.3$ s) and a high mass WD (${\approx} 0.9 M_\odot$), or a solitary millisecond pulsar (MSP).