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The massive star binary fraction in young open clusters – I. NGC 6231 revisited
Authors:H Sana  E Gosset †  Y Nazé †  G Rauw †  N Linder †‡
Institution:European Southern Observatory, Alonso de Cordova 1307, Casilla 19001, Santiago 19, Chile;Astrophysical Institute, Liège University, Bat. B5c, Allée du 6 Août 17, B-4000 Liège, Belgium
Abstract:We present the results of a long-term high-resolution spectroscopy campaign on the O-type stars in NGC 6231. We revise the spectral classification and multiplicity of these objects and we constrain the fundamental properties of the O-star population. Almost three quarters of the O-type stars in the cluster are members of a binary system. The minimum binary fraction is 0.63, with half the O-type binaries having an orbital period of the order of a few days. The eccentricities of all the short-period binaries are revised downward, and henceforth match a normal period–eccentricity distribution. The mass ratio distribution shows a large preference for O + OB binaries, ruling out the possibility that, in NGC 6231, the companion of an O-type star is randomly drawn from a standard initial mass function. Obtained from a complete and homogeneous population of O-type stars, our conclusions provide interesting observational constraints to be confronted with the formation and early evolution theories of O-stars.
Keywords:binaries: close  binaries: general  binaries: spectroscopic  stars: early-type  open clusters and associations: individual: NGC 6231  open clusters and associations: individual: Sco OB1
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