Preprint Logo P105 April
2006


To be published in:
Astronomy & Astrophysics

An XMM-Newton view of the young open cluster NGC~6231+
I. The catalogue

H. Sana1,*, E. Gosset1,**, G. Rauw1,**, H. Sung2 and J.-M. Vreux1


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 collected with XMM-Newton, an ESA Science Mission with instruments and contributions directly funded by ESA Member states and the USA (NASA).


Abstract

     This paper is the first of a series dedicated to the X-ray properties of the young open cluster NGC 6231. Our data set relies on an XMM-Newton campaign of a nominal duration of 180ks and reveals that NGC 6231 is very rich in the X-ray domain too. Indeed, 610 X-ray sources are detected in the present field of view, centered on the cluster core. The limiting sensitivity of our survey is approximately 6 x 10-15 erg cm-2 s-1 but clearly depends on the location in the field of view and on the source spectrum. Using different existing catalogues, over 85% of the X-ray sources could be associated with at least one optical and/or infrared counterpart within a limited cross-correlation radius of 3" at maximum. The surface density distribution of the X-ray sources presents a slight N-S elongation. Once corrected for the spatial sensitivity variation of the EPIC instruments, the radial profile of the source surface density is well described by a King profile with a central density of about 8 sources per arcmin2 and a core radius close to 3.1 arcmin. The distribution of the X-ray sources seems closely related to the optical source distribution. The expected number of foreground and background sources should represent about 9% of the detected sources, thus strongly suggesting that most of the observed X-ray emitters are physically belonging to NGC 6231. Finally, beside a few bright but soft objects -- corresponding to the early-type stars of the cluster -- most of the sources are relatively faint (~ 5 x 10-15 erg cm-2 s-1) with an energy distribution peaked around 1.0-2.0 keV.

Key Words
Open clusters and associations: individual: NGC 6231 -- X-rays: individuals: NGC 6231 -- X-rays: stars -- Stars: early-type -- Catalogues

Files
Article: 3224.pdf (1077157 bytes): HTTP FTP (ftp://vela.astro.ulg.ac.be/preprints/)


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