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Paper: Mass-loss Rates for Very Massive Stars Up to 300 Solar Masses
Volume: 440, UP2010: Have Observations Revealed a Variable Upper End of the Initial Mass Function?
Page: 83
Authors: Vink, J. S.
Abstract: One of the key questions in Astrophysics concerns the issue of whether there exists an upper mass limit to stars and if so, what physical mechanism determines this upper limit. Here we present the latest mass-loss predictions for the most massive stars in our Universe – in the mass range up to 300 solar masses – using a novel hydrodynamic method that includes the important effects of multiple photons interactions, allowing us to predict the rate of mass loss and the wind terminal velocity simultaneously. Our model stars have a high Eddington factor (Γ) and we find an upturn in the mass-loss versus Γ dependence, where the model winds become optically thick. This is also the point where our wind efficiency numbers – defined as the wind momentum over the photon momentum – surpass the single-scattering limit (of η = 1), reaching wind efficiency numbers up to η ≃ 2.5. Our modelling indicates a natural transition from common O-type stars to Wolf-Rayet characteristics when the wind becomes optically thick. This “transitional'' behaviour is also reflected in the wind acceleration parameter β, which naturally reaches values as high as 1.5-2, as well as in the spectral morphology of the He II line at 4686Å – characteristic for Of and late WN stars. In Wolf-Rayet galaxy research, the feature is sometimes referred to as “the blue bump''.
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