Rotating models of very massive close binary stars at solar metallicity: Evolution and remnant masses
Abstract
To investigate the evolution of massive close binary systems at solar metallicity, we utilized the Modules for Experiments in Stellar Astrophysics (MESA) numerical code to compute a grid of 2,880 detailed evolutionary models. These models consist of rotating stars with initial masses between 100 and 250 ̊m M_⊙ and initial orbital periods of three to ten days. We find that, with the exception of a small fraction of systems in contact on the zero-age main sequence (ZAMS), the binaries with standard stellar wind mass-loss rates reach core collapse without undergoing mass transfer. The primary stars remain relatively compact as their radii do not exceed ≈ 20 ̊m R_⊙. The evolution is dominated by extreme stellar wind mass loss, enhanced by rapid rotation driven by tidal synchronization. As surface hydrogen is heavily depleted during the main sequence, the stars transition into the optically thick Wolf--Rayet wind regime, ultimately shedding 90% to 95% of their initial mass. Consequently, increasing the initial stellar mass does not yield more massive remnants; all resulting black holes (BHs) fall within a narrow range of 7--14 ̊m M_⊙. This severe mass loss causes significant orbital widening, ensuring that the resulting double BH systems will not merge within a Hubble time. We conclude that presented evolutionary models with standard stellar wind mass-loss rates result in BHs less massive than ≈15 ̊m M_⊙ in very wide configurations. We also show that if mass-loss rates are scaled down by a factor of two or more, the same initial binary configurations can form (sometimes via mass transfer) significantly closer double BHs with masses exceeding 50 ̊m M_⊙ – this matches the upper range of LIGO-Virgo-KAGRA (LVK) observations. Corresponding maximum stellar radii were restricted to ≈ 25 and ≈ 65 for wind mass-loss reductions of 50% and 10%, respectively. Although excluded from our standard wind models, secondary-to-BH mass transfer or a common envelope phase becomes viable when assuming reduced mass-loss rates. These binary interactions would substantially tighten the orbit, potentially producing double BH binaries close enough to merge within a Hubble time.