To be published in:
ASP Conference Series
NGC 6231: X-ray properties of the early-type star population +
H. Sung2 and
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).
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 LX/Lbol
relationship and we confirm the clear dichotomy between O- and B-type stars.
The cut-off line between the two behaviours occurs at
Lbol ~ 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