The R136 star cluster hosts several stars whose individual masses greatly exceed the accepted 150Msolar stellar mass limit

Spectroscopic analyses of hydrogen-rich WN5-6 stars within the young star clusters NGC3603 and R136 are presented, using archival Hubble Space Telescope and Very Large Telescope spectroscopy, and high spatial resolution near-IR photometry, including Multi-Conjugate Adaptive Optics Demonstrator (MAD) imaging of R136. We derive high stellar temperatures for the WN stars in NGC3603 (T_* ~ 42 +/- 2kK) and R136 (T_* ~ 53 +/- 3kK) plus clumping-corrected mass-loss rates of 2-5 × 10^-5M_solaryr^-1 which closely agree with theoretical predictions from Vink et al. These stars make a disproportionate contribution to the global ionizing and mechanical wind power budget of their host clusters. Indeed, R136a1 alone supplies ~7 per cent of the ionizing flux of the entire 30Doradus region. Comparisons with stellar models calculated for the main-sequence evolution of 85-500M_solar accounting for rotation suggest ages of ~1.5Myr and initial masses in the range 105-170M_solar for three systems in NGC3603, plus 165-320M_solar for four stars in R136. Our high stellar masses are supported by consistent spectroscopic and dynamical mass determinations for the components of NGC3603A1. We consider the predicted X-ray luminosity of the R136 stars if they were close, colliding wind binaries. R136c is consistent with a colliding wind binary system. However, short period, colliding wind systems are excluded for R136a WN stars if mass ratios are of order unity. Widely separated systems would have been expected to harden owing to early dynamical encounters with other massive stars within such a high-density environment. From simulated star clusters, whose constituents are randomly sampled from the Kroupa initial mass function, both NGC3603 and R136 are consistent with an tentative upper mass limit of ~300M_solar. The Arches cluster is either too old to be used to diagnose the upper mass limit, exhibits a deficiency of very massive stars, or more likely stellar masses have been underestimated - initial masses for the most luminous stars in the Arches cluster approach 200M_solar according to contemporary stellar and photometric results. The potential for stars greatly exceeding 150M_solar within metal-poor galaxies suggests that such pair-instability supernovae could occur within the local universe, as has been claimed for SN2007bi.