The Tarantula Nebula in the Large Magellanic Cloud is our closest view of a starburst region and is the ideal environment to investigate important questions regarding the formation, evolution and final fate of the most massive stars. We analyze the multiplicity properties of the massive O-type star population observed through multi-epoch spectroscopy in the framework of the VLT-FLAMES Tarantula Survey. With 360 O-type stars, this is the largest homogeneous sample of massive stars analyzed to date. We use multi-epoch spectroscopy and variability analysis to identify spectroscopic binaries. We also use a Monte-Carlo method to correct for observational biases. By modelling simultaneously the observed binary fraction, the distributions of the amplitudes of the radial velocity variations and the distribution of the time scales of these variations, we derive the intrinsic current binary fraction and period and mass-ratio distributions. We observe a spectroscopic binary fraction of 0.35+/-0.03, which corresponds to the fraction of objects displaying statistically significant radial velocity variations with an amplitude of at least 20km/s . We compute the intrinsic binary fraction to be 0.51+/-0.04. We adopt power-laws to describe the intrinsic period and mass-ratio distributions: f(log_10_P/d)~(log_10_ P/d)^pi^ (with log_10_P/d in the range 0.15-3.5) and f(q)~q^kappa^ with 0.115.5mag), which results from observational effects and the fact that our O star sample is not magnitude-limited but is defined by a spectral-type cutoff. We also conclude that magnitude-limited investigations are biased towards larger binary fractions. Using the multiplicity properties of the O stars in the Tarantula region and simple evolutionary considerations, we estimate that over 50% of the current O star population will exchange mass with its companion within a binary system. This shows that binary interaction is greatly affecting the evolution and fate of massive stars, and must be taken into account to correctly interpret unresolved populations of massive stars.

Cone search capability for table J/A+A/550/A107/stars (Stars observed: list of lines used for RV measurements and Multiplicity status)