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The most massive stars are thought to be hydrogen-rich Wolf-Rayet stars oflate spectral subtype (WNh stars). In previous theoretical studies the enhancedmass loss of these stars has been attributed to their proximity to theEddington limit. Here we investigate observed trends in the mass-lossproperties of such young, very massive stars. We derive theoreticalmass-luminosity relations for very massive stars, based on a large grid ofstellar structure models. Using these relations, we estimate Eddington factorsfor a sample of stars, under different assumptions of their evolutionarystatus. We evaluate the resulting mass-loss relations, and compare them withtheoretical predictions. We find observational evidence that the mass loss inthe WR regime is dominated by the Eddington parameter Gamma_e, which hasimportant consequences for the way we understand Wolf-Rayet stars and theirmass loss. In addition, we derive wind masses that support the picture that theWNh stars in young stellar clusters are very massive, hydrogen-burning stars.Our findings suggest that the proximity to the Eddington limit is the physicalreason for the onset of Wolf-Rayet type mass loss. This means that, e.g. instellar evolution models, the Wolf-Rayet stage should be identified by largeEddington parameters, instead of a helium-enriched surface composition. Thelatter is most likely only a consequence of strong mass loss, in combinationwith internal mixing. For very massive stars, the enhanced Gamma-dependent massloss is responsible for the formation of late WNh subtypes with high hydrogensurface abundances, partly close to solar. Because mass loss dominates theevolution of very massive stars, we expect a strong impact of this effect ontheir end products, in particular on the potential formation of black holes,and Gamma-Ray Bursts, as well as the observed upper mass limit of stars.

The Eddington factor as the key to understand the winds of the most massive stars. Evidence for a Gamma-dependence of Wolf-Rayet type mass loss