Spectroscopic analyses of hydrogen-rich WN 5-6 stars within the young star clusters NGC 3603 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 NGC 3603 (T * ∼ 42 ± 2 kK) and R136 (T * ∼ 53 ± 3 kK) plus clumping-corrected mass-loss rates of 2-5 × 10 −5 M yr −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 30 Doradus region. Comparisons with stellar models calculated for the main-sequence evolution of 85-500 M accounting for rotation suggest ages of ∼1.5 Myr and initial masses in the range 105-170 M for three systems in NGC 3603, plus 165-320 M for four stars in R136. Our high stellar masses are supported by consistent spectroscopic and dynamical mass determinations for the components of NGC 3603A1. 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 NGC 3603 and R136 are consistent with an tentative upper mass limit of ∼300 M . 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 200 M according to contemporary stellar and photometric results. The potential for stars greatly exceeding 150 M within metal-poor galaxies suggests that such pair-instability supernovae could occur within the local universe, as has been claimed for SN 2007bi.

We discuss the observations and theory of star cluster formation to argue that clusters form dynamically cool (subvirial) and with substructure. We then perform an ensemble of simulations of cool, clumpy (fractal) clusters and show that they often dynamically mass segregate on timescales far shorter than expected from simple models. The mass segregation comes about through the production of a short-lived, but very dense core. This shows that in clusters like the Orion Nebula Cluster the stars ≥ 4M ⊙ can dynamically mass segregate within the current age of the cluster. Therefore, the observed mass segregation in apparently dynamically young clusters need not be primordial, but could be the result of rapid and violent early dynamical evolution.

Observations and theory both suggest that star clusters form sub-virial (cool) with highly sub-structured distributions. We perform a large ensemble of N-body simulations of moderate-sized (N=1000) cool, fractal clusters to investigate their early dynamical evolution. We find that cool, clumpy clusters dynamically mass segregate on a short timescale, that Trapezium-like massive higher-order multiples are commonly formed, and that massive stars are often ejected from clusters with velocities > 10 km/s (c.f. the average escape velocity of 2.5 km/s). The properties of clusters also change rapidly on very short timescales. Young clusters may also undergo core collapse events, in which a dense core containing massive stars is hardened due to energy losses to a halo of lower-mass stars. Such events can blow young clusters apart with no need for gas expulsion. The warmer and less substructured a cluster is initially, the less extreme its evolution.Comment: Accepted for publication in MNRAS; supplementary material can be downloaded at http://sgoodwin.staff.shef.ac.uk/all_plots.pdf.g

We present a new method to detect and quantify mass segregation in star clusters. It compares the minimum spanning tree (MST) of massive stars with that of random stars. If mass segregation is present, the MST length of the most massive stars will be shorter than that of random stars. This difference can be quantified (with an associated significance) to measure the degree of mass segregation. We test the method on simulated clusters in both 2D and 3D and show that the method works as expected. We apply the method to the Orion Nebula Cluster (ONC) and show that the method is able to detect the mass segregation in the Trapezium with a `mass segregation ratio' \Lambda_{MSR}=8.0 \pm 3.5 (where \Lambda_{MSR}=1 is no mass segregation) down to 16 \Msun, and also that the ONC is mass segregated at a lower level (~2.0 \pm 0.5) down to 5 \Msun. Below 5 \Msun we find no evidence for any further mass segregation in the ONC.Comment: Accepted in MNRA

We examine substructure and mass segregation in the massive OB association Cygnus OB2 to better understand its initial conditions. Using a well understood Chandra X-ray selected sample of young stars we find that Cyg OB2 exhibits considerable physical substructure and has no evidence for mass segregation, both indications that the association is not dynamically evolved. Combined with previous kinematical studies we conclude that Cyg OB2 is dynamically very young, and what we observe now is very close to its initial conditions: Cyg OB2 formed as a highly substructured, unbound association with a low volume density (< 100 stars pc −3 ). This is inconsistent with the idea that all stars form in dense, compact clusters. The massive stars in Cyg OB2 show no evidence for having formed particularly close to one another, nor in regions of higher than average density. Since Cyg OB2 contains stars as massive as ∼100 M ⊙ this result suggests that very massive stars can be born in relatively low-density environments. This would imply that massive stars in Cyg OB2 did not form by competitive accretion, or by mergers.