AGES OF YOUNG STAR CLUSTERS, MASSIVE BLUE STRAGGLERS, AND THE UPPER MASS LIMIT OF STARS: ANALYZING AGE-DEPENDENT STELLAR MASS FUNCTIONS

Massive stars rapidly change their masses through strong stellar winds and mass transfer in binary systems. The latter aspect is important for populations of massive stars as more than 70% of all O stars are expected to interact with a binary companion during their lifetime. We model the observed present-day mass functions of the young Galactic Arches and Quintuplet star clusters using our rapid binary evolution code. We find that the shaping of the mass function by stellar wind mass loss allows us to determine the cluster ages as 3.5 + or - 0.7 Myr and 4.8 + or - 1.1 Myr, respectively. Our findings have strong implications for the stellar upper mass limit and solve the discrepancy between the claimed 150 M sub([middot in circle]) limit and observations of four stars with initial masses of 165-320 M sub([middot in circle]) in R136 and of supernova 2007bi, which is thought to be a pair-instability supernova from an initial 250 M sub([middot in circle]) star.