The effects of binaries on the evolution of Wolf-Rayet type spectral features in starbursts

In the present paper we investigate in detail the effects of binaries with initial period between 1 day and 10 years on theoretical simulations of Wolf-Rayet (WR) type spectral features in starbursts. We focus on the evolution of the nebular H-beta line in starburst in general, on the intensity ratios I(nebular He II lambda4686)/I(H-beta), I(blue bump)/I(H-beta), and I(red bump)/I(H-beta) as a function of the equivalent width of H-beta of WR galaxies in particular. The binary evolutionary processes that dominate the evolution of the considered spectral features are the Roche lobe overflow in case Br systems, the mass transfer efficiency, and the merger rate. We show that the predictions on nebular He II depend critically on the uncertainties in the theory of WR atmospheres and particularly on uncertainties in the treatment of the subsonic velocity region of the WR wind. The observations of low metallicity starbursts are best reproduced by a theoretical model with a significant number of binaries and with a metallicity-dependent WR wind.