P97	November 2004

¹ Institut d'Astrophysique et de Géophysique - Université de Liège, Allée du 6 Août, Bât B5c, B-4000 Liège (Sart Tilman), Belgium
² Department of Astronomy and Space Science, Sejong University, Kunja-dong 98, Kwangjin-gu, Seoul 143-747, Korea.

^* Research Fellow FNRS, Belgium
^** Research Associate FNRS, Belgium

⁺ Based on observations with XMM-Newton, an ESA Science Mission with instruments and contributions directly funded by ESA Member States and the USA (NASA).

Abstract

     Based on a deep XMM-Newton observation of the young open cluster NGC 6231, we derive the main X-ray properties of its early-type star population. Among the 610 X-ray sources detected in the field, 42 are associated with early-type stars. We investigate their L_X/L_bol relationship and we confirm the clear dichotomy between O- and B-type stars. The cut-off line between the two behaviours occurs at L_bol ~ 10³⁸ erg s^-1 as previously proposed by Bergöfer et al. (1997). The distinction between single and binary stars is not clear cut, except for the colliding wind system HD 152248. The X-ray detected B-stars in NGC 6231 appear to be more luminous than predicted from the Bergvfer et al. relation. Though this suggests a bimodal distribution of the B-star X-ray emission, we caution however that these results might be biased by detection limits. Finally we investigate the X-ray variability of the detected sources and we find that about 40% of the X-ray emitters in the field of view present consistent signs of variability in the EPIC instruments. This fraction is much larger than previously thought. About one third of the early-type stars population, either singles or binaries, further shows variability. These variations do not seem to be exclusively related to binarity and could thus not be systematically interpreted in the framework of a wind-wind collision phenomenon.