SCYON Abstract

Received on January 5 2001

High-mass binaries in the very young open cluster NGC 6231: Implication for cluster and star formation

Authors	Beatriz Garcia¹ Jean-Claude Mermilliod²
Affiliation	¹CRICyT- Centro Regional de Investigaciones Cienti ficas y Tecnicas. CC 131 - 5500 Mendoza, Argentina. ²Institut d'Astronomie de l'Université de Lausanne, CH-1290 Chavannes-des-Bois, Switzerland
Accepted by	Astronomy & Astrophysics
Contact	Jean-Claude.Mermilliod@obs.unige.ch
URL	http://obswww.unige.ch/~mermio/ia/preprints.html
Links	NGC 6231

Abstract

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.