  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.
