1995A&A...298..767M

We present new evolutionary population synthesis models based on the most recent grids of stellar models computed at the Geneva Observatory. We study the effects on the massive star populations born in a starburst, of the star formation rate (SFR), of the initial mass function (IMF), of the age, of the metallicity (Z) and of a change of the mass loss rates by stellar winds ({dot}(M)). We obtain that the more intense and shorter is the burst of star formation, the higher are the ratios of WR to O-type stars reached after the burst. The same trend is expected when the IMF's slope becomes flatter, the upper mass cut off, the metallicity, and/or the mass loss rates increase. At a given age and metallicity, the way the WR are distributed among the different WR subtypes depends sensitively on the rate of mass loss experienced by the stars. For metallicities Z=0.004 and Z=0.008, we have that only the high {dot}(M) models do predict the existence of a long WC-dominated phase. Moreover, only the high {dot}(M) models might account for the presence of a significant number of WC stars at very low metallicity (Z=0.001). We estimate the percentage of young starbursts, (i.e. with O and/or WR stars), whose massive star population is dominated by O-type stars, Wolf-Rayet, WNL, WNE and WC stars respectively. We find that the fraction of starbursts with a WNL-dominated population varies between 6 and 33% depending on the metallicity. We study the supernova rates expected in recent and powerful star formation regions and find that the maximum rate occurs when the most massive stars explode. One obtains that for the most powerful starburst region observed by Vacca & Conti (1992ApJ...401..543V, NGC 1614), one can expect, at the maximum, a supernova rate of about 12 supernovae per century. The present evolutionary population synthesis models can account for the very high ratios of WNL to O-type stars observed in some starburst galaxies. If the presence of a significant WC star population is confirmed in the low metallic starburst regions He 2-10 A and NGC 4214, this will be an indication that the mass loss rates experienced by the stars, at least during the pre-WR phases, are higher than previously thought, in agreement with the conclusions of Maeder & Meynet (1994A&A...287..803M).