The Pre-Roche Lobe Overflow Evolution of Massive Close Binary Stars: A Study of Rotation, Wind Enhanced Mass-Loss, and the Bi-Stability Jump

Massive stars have the ability to enrich their environment with heavy elements and influence star formation in galaxies. Some massive stars exist in binary systems with short orbital periods. These are called massive close binaries. It is important to understand the evolution of massive close binary systems to gain insight about galaxy evolution. Massive stars above 20 solar masses experience a bi-stability jump where there is a sudden increase in mass-loss rate in their winds. There is ongoing research in this field, but the study of the bi-stability jump and its effects on massive close binary star properties has not been done before. A related question is whether binarity can produce a slow rotating, nitrogen-rich massive star such as those found in the Large Magellanic Cloud (Hunter et al. 2008). To accomplish this, two single-star models from Higgins & Vink (2019) and Brott et al. (2011) were used to model a close binary system with the 1-dimensional hydrodynamic stellar evolution code MESA. A grid of models using Higgins & Vink (2019) stellar parameters was created by varying 5 parameters: the convective step overshoot, the tidally enhanced wind coefficient, the wind enhancement factor, the initial rotation, and the initial masses of both stars. Two models were created to compare the approaches of Higgins & Vink (2019) and Brott et al. (2011). Results show that early on in the evolution of the rotating models, the primary star has a more nitrogen-rich photosphere and rotates slower than the secondary star. Tidally enhanced winds are strong enough to strip off the surface layers of the primary. This exposed the nitrogen-rich envelope that is enhanced due to mixing. Tidal forces and tidally enhanced winds slow the rotation rate of the primary star. The existence of the bi-stability jump in massive close binary stars does have an effect on binary properties and could prevent a Roche lobe overflow event. From the numerical data from the models, predictions for characteristics of a wind-blown bubble provide possible future observational properties that are testable with current X-ray observatories