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Some studies have claimed a universal stellar upper-mass limit of 150 Msun. Afactor that is often overlooked is that there might be a difference between thecurrent and initial masses of the most massive stars, as a result of mass loss.We present Monte Carlo mass-loss predictions for very massive stars in therange 40-300 Msun, with large luminosities and Eddington factors Gamma. Usingour new dynamical approach, we find an upturn in the mass-loss vs. Gammadependence, at the point where the winds become optically thick. This coincideswith the location where wind efficiency numbers surpass the single-scatteringlimit of Eta = 1, reaching values up to Eta = 2.5. Our modelling suggests atransition from common O-type winds to Wolf-Rayet characteristics at the pointwhere the winds become optically thick. This transitional behaviour is alsorevealed with respect to the wind acceleration parameter beta, which starts atvalues below 1 for the optically thin O-stars, and naturally reaches values ashigh as 1.5-2 for the optically thick Wolf-Rayet models. An additional findingconcerns the transition in spectral morphology of the Of and WN characteristicHe II line at 4686 Angstrom. When we express our mass-loss predictions as afunction of the electron scattering Gamma_e (=L/M) only, we obtain a mass-lossGamma dependence that is consistent with a previously reported power-law Mdotpropto Gamma^5 (Vink 2006) that was based on our semi-empirical modellingapproach. When we express Mdot in terms of both Gamma and stellar mass, we findMdot propto M^0.8 Gamma^4.8 for our high Gamma models. Finally, we confirm thatthe Gamma-effect on the mass-loss predictions is much stronger than that of anincreased helium abundance, calling for a fundamental revision in the way massloss is incorporated in evolutionary models of the most massive stars.

Wind modelling of very massive stars up to 300 solar masses