The massive star binary fraction in young open clusters - III. IC 2944
  and the Cen OB2 association
  Using an extended set of multi-epoch high-resolution high signal-to-noise
ratio optical spectra, we readdress the multiplicity properties of the O-type
stars in IC 2944 and in the Cen OB2 association. We present new evidence of
binarity for five objects and we confirm the multiple nature of another two. We
derive the first orbital solutions for HD 100099, HD 101436 and HD 101190 and
we provide additional support for HD 101205 being a quadruple system. The
minimal spectroscopic binary fraction in our sample is f_min = 0.57. Using
numerical simulations, we show that the detection rate of our observational
campaign is close to 90 per cent, leaving thus little room for undetected
spectroscopic binary systems. The statistical properties of the O-star
population in IC 2944 are similar, within the uncertainties, to the results
obtained in the earlier papers in this series despite the fact that sample size
effects limit the significance of the comparison. Using newly derived
spectroscopic parallaxes, we reassess the distance to IC 2944 and obtained
2.3+/-0.3 kpc, in agreement with previous studies. We also confirm that, as far
as the O stars are concerned, the IC 2944 cluster is most likely a single
entity.
