An XMM-Newton view of the young open cluster NGC 6231 - III. Optically faint X-ray sources

We discuss the properties of the X-ray sources with faint optical counterparts in the very young open cluster NGC 6231. From their positions in the Hertzsprung-Russell diagram, we find that the bulk of these objects probably consists of low-mass pre-main-sequence (PMS) stars with masses in the range 0.3-3.0 M_solar. The age distribution of these objects indicates that low-mass star formation in NGC 6231 started more than 10Myr ago and culminated in a starburst-like event about 1-4Myr ago when the bulk of the low-mass PMS stars as well as the massive cluster members formed. We find no evidence for a spatial age gradient that could point towards a sequential star formation process. Only a few X-ray sources have counterparts with a reddening exceeding the average value of the cluster or with infrared colours indicating the presence of a moderate near-IR excess. The X-ray spectra of the brightest PMS sources are best fitted by rather hard thermal plasma models and a significant fraction of these sources display flares in their light curve. The X-ray brightest flaring sources have decay times between 2 and 16ks. The X-ray selected PMS stars in NGC 6231 have logL_X/L_bol values that increase strongly with decreasing bolometric luminosity and can reach a saturation level (logL_X/L_bol ~ -2.4) for non-flaring sources and even more extreme values during flares.

Based on observations with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA member states and the USA (NASA).

E-mail: hsana@eso.org ‡

FNRS Research Associate (Belgium).