Open AccessAn XMM–Newton view of the young open cluster NGC 6231 – III. Optically faint X‐ray sources ★
Author(s) -
Sana H.,
Rauw G.,
Sung H.,
Gosset E.,
Vreux J.M.
Publication year - 2007
Publication title -
monthly notices of the royal astronomical society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.058
H-Index - 383
eISSN - 1365-2966
pISSN - 0035-8711
DOI - 10.1111/j.1365-2966.2007.11672.x
Subject(s) - physics , astrophysics , open cluster , stars , luminosity , astronomy , light curve , cluster (spacecraft) , hertzsprung–russell diagram , star cluster , stellar evolution , galaxy , computer science , programming language
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 ⊙ . The age distribution of these objects indicates that low‐mass star formation in NGC 6231 started more than 10 Myr ago and culminated in a starburst‐like event about 1–4 Myr 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 16 ks. The X‐ray selected PMS stars in NGC 6231 have log L X / L bol values that increase strongly with decreasing bolometric luminosity and can reach a saturation level (log L X / L bol ∼−2.4) for non‐flaring sources and even more extreme values during flares.