The massive star binary fraction in young open clusters – I. NGC 6231 revisited |
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Authors: | H Sana E Gosset † Y Nazé † G Rauw † N Linder †‡ |
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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 |
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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. |
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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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