Evolutionary models for the very massive stars in the R136 cluster of 30 Doradus in the Large Magellanic Cloud

Context. The cluster Radcliffe 136 in the Large Magellanic Cloud (LMC) contains a population of stars in excess of 100 M_⊙, including the most massive star known, R136a1. Very massive stars (VMSs) play an influential role in feedback processes and may potentially produce exotic supernova (SN) types and black holes of tens of solar masses.
Aims. The evolutionary history and final fate of the three most luminous stars, R136a1, R136a2, and R136a3, continues to be a puzzling issue. We aim to resolve this using dedicated stellar evolution models.
Methods. We computed rotating single-star MESA models and applied observationally constrained mass-loss rates during the early evolution and new theoretical Wolf-Rayet-type rates once the surface becomes enriched in helium. We considered various scenarios for internal angular momentum (AM) transport. We produced interpolated model grids and applied a Markov chain Monte Carlo (MCMC) analysis to compare our models with observations.
Results. The nature of SN progenitors strongly depends on mass loss and the AM coupling schemes. We predict no pair-instability and no gamma-ray burst progenitors from our fiducial model grid at LMC metallicity. The onset of Wolf-Rayet-type mass-loss rates on the main sequence leads to a rapid decrease in stellar mass and luminosity. The initially most massive model (800 M_⊙) loses mass the most rapidly and becomes less massive than the initially least massive model (100 M_⊙) in our grid. This mass turnover implies that the evolutionary history can only be inferred if additional constraints are available. We utilised the surface helium abundance, which poses a conundrum: R136a1, the most luminous star, is less enriched in helium than R136a2 and R136a3. We propose that this can be explained if both R136a2 and R136a3 were initially more massive than R136a1. From a rigorous confrontation of our models to spectroscopically derived observables, we estimate an initial mass of 346 ± 42 M_⊙ for R136a1, and ≳500 M_⊙ for R136a2 and R136a3.
Conclusions. Even though VMSs are only present in the youngest clusters below 2 Myr of age, our study demonstrates the greater strength of their role in local and galaxy evolution. At the LMC metallicity, they will be observable as helium-enriched massive stars after their drastic mass loss, produced via single-star evolution. If the core collapse leads to a SN, it will be of Type Ib/c.