Program: GS-2005B-Q-2

With the purpose of comparing the dynamical and the photometric masses of the ionising cluster of 30 Doradus we used the MOS mode of EMMI to determine the velocity dispersion sigma_v of the brightest stars in the central part of the cluster. We obtained sigma_v = 32 km/s which is much larger than the value predicted from a photometric determination of the cluster's mass. The most plausible explanation for this unexpected result is that a large fraction of the stars in the cluster are binaries. This proposal intends to use GMOS in its multi-object spectroscopic mode to measure the radial velocity of a sample of ~80 stars of spectral types earlier than B1 in the central cluster of 30 Doradus. By measuring these velocities in 4 different epochs we propose to identify all binaries and to measure the true value of sigma_v. The interesting issue of this experiment is that both possible outcomes yield useful information: If the binary fraction is as large as we infer from our NTT observations, the implications for our understanding of starburst formation and evolution will be extremely important, and if binaries are not detected, then our measured value of stellar velocity dispersion will be confirmed and this poses tough constraints on the dynamics of starburst clusters.