1996A&A...307..829B

The formation of stars up to 120M_☉ is computed in the framework of the accretion scenario. For realistic accretion rates derived from the observed line width in various molecular dark clouds, the accretion interlude lasts some 2-2.5Myr, and accounts for an appreciable nuclear evolution during the optically thick MS life. Several new results are found concerning massive stars at the top of the MS: 1) A newly formed massive star with M≥40M_☉ at the time it emerges from its parental cloud has already burned a substantial fraction of its central hydrogen content. 2) As a consequence, the formal MS lifetime is substantially reduced. 3) A proper ZAMS does not exist, since at the time it becomes visible, the star has already evolved towards lower T_eff. 4) As a result of previous evolution, the size of the convective core for a given central H content is reduced by about 5-10%. 5) We find that for realistic accretion rates applicable to ordinary star forming regions in the Galaxy and Magellanic Clouds, a truncation of the IMF is naturally established around 85-150M_☉ where the accretion time becomes comparable to the hydrogen burning time. 6) Since massive stars spend a fraction of their H-burning phase in the parental cocoon, their true number is larger than estimated and the slope of the IMF is flatter.