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Physics and evolution of the most massive stars in 30 Dor. Mass loss, envelope inflation, and a variable upper stellar mass limit

The identification of stellar-mass black-hole mergers with up to 80 Msun as powerful sources of gravitational wave radiation led to increased interest in the physics of the most massive stars. The largest sample of possible progenitors of such objects, very massive stars (VMS) with masses up to 300 Msun, have been identified in the 30 Dor star-forming region in the Large Magellanic Cloud (LMC). The physics and evolution of VMS is highly uncertain, mainly due to their proximity to the Eddington limit. In this work we investigate the two most important effects that are thought to occur near the Eddington limit. Enhanced mass loss through optically thick winds, and the formation of radially inflated stellar envelopes. We compute evolutionary models for VMS at LMC metallicity and perform a population synthesis of the young stellar population in 30 Dor. We find that enhanced mass loss and envelope inflation have a dominant effect on the evolution of the most massive stars. While the observed mass-loss properties and the associated surface He-enrichment are well described by our new models, the observed O-star mass-loss rates are found to cover a much larger range than theoretically predicted, with particularly low mass-loss rates for the youngest objects. Also, the (rotational) surface enrichment in the O-star regime appears to be not well understood. The positions of the most massive stars in the Hertzsprung-Russell Diagram (HRD) are affected by mass loss and envelope inflation. For instance, the majority of luminous B-supergiants in 30 Dor, and the lack thereof at the highest luminosities, can be explained through the combination of envelope inflation and mass loss. Finally, we find that the upper limit for the inferred initial stellar masses in the greater 30 Dor region is significantly lower than in its central cluster R 136, implying a variable upper limit for the masses of stars.