EDP Sciences
Free Access
Volume 368, Number 1, March II 2001
Page(s) 122 - 136
Section Stellar clusters and associations
DOI https://doi.org/10.1051/0004-6361:20000528

A&A 368, 122-136 (2001)
DOI: 10.1051/0004-6361:20000528

High-mass binaries in the very young open cluster NGC 6231

Implication for cluster and star formation
B. García1 and J. C. Mermilliod2

1  CRICyT- Centro Regional de Investigaciones Científicas y Técnicas, CC 131-5500 Mendoza, Argentina
2  Institut d'Astronomie de l'Université de Lausanne, 1290 Chavannes-des-Bois, Switzerland

(Received 3 October 2000 / Accepted 20 December 2000)

New radial-velocity observations of 37 O- and B stars in the very young open cluster NGC 6231 confirm the high frequency of short-period spectroscopic binaries on the upper main sequence. Among the 14 O-type stars, covering all luminosity classes from dwarfs to supergiants, 8 are definitively double-lined systems and all periods but one are shorter than 7 days. Several additional binaries have been detected among the early B-type stars. NGC 6231 is an exceptional cluster to constrain the scenarios of cluster- and binary-star formation over a large range of stellar masses. We discuss the evidences, based on NGC 6231 and 21 other clusters, with a total of 120 O-type stars, for a clear dichotomy in the multiplicity rate and structure of very young open clusters containing O-type stars in function of the number of massive stars. However, we cannot answer the question whether the observed characteristics result from the formation processes or from the early dynamical evolution.

Key words: galaxies: open clusters and associations: NGC 6231 -- stars: binaries: spectroscopic -- stars: early-type -- stars: rotation

Offprint request: B. García, bgarcia@lab.cricyt.edu.ar

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© ESO 2001

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