The massive stars in the R136 region of 30 Doradus.

Abstract

We present U and B photometry, based on Hubble Space Telescope observations, for 26 stars in the R136 region of the young cluster 30 Doradus in the Large Magellanic Cloud. It has been suggested that rich star clusters form through the coalescence of clumps, and it seems likely that the R136 region is such a clump. If it is in virial equilibrium then its mass (~1500 M_sun_) and radius (~0.8 pc) imply a velocity dispersion of ~3 km s^-1^. The three brightest stars in the clump may have masses as large as ~250 M_sun_, but given the uncertainties, a value this large is not required by the data. Two of these stars have a projected separation of only ~4500 AU, and may constitute a hard binary with orbital velocity ~12.5+/-1.5 km s^-1^. N-body simulations suggest that mass loss through stellar evolution would unbind the clump after only a few million years, and that it is unlikely that a hard binary could form dynamically in that time. We therefore conclude that, if the two massive stars are a binary, it is primordial.