stars : pre-main-sequence; open clusters and associations : individual : NGC 6231; X-rays : individual : NGC 6231; X-rays : stars
Abstract :
[en] 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-circle dot. 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 similar to -2.4) for non-flaring sources and even more extreme values during flares.
Disciplines :
Earth sciences & physical geography
Author, co-author :
Sana, H.
Rauw, Grégor ; Université de Liège - ULiège > Département d'astrophys., géophysique et océanographie (AGO) > Groupe d'astrophysique des hautes énergies (GAPHE)
Sung, H.
Gosset, Eric ; Université de Liège - ULiège > Département d'astrophys., géophysique et océanographie (AGO) > Groupe d'astrophysique des hautes énergies (GAPHE)
Vreux, Jean-Marie ; Université de Liège - ULiège > Département d'astrophysique
Language :
English
Title :
An XMM-Newton view of the young open cluster NGC 6231-III. Optically faint X-ray sources
Bohlin R. C., Savage B. D., Drake J. F., 1978, ApJ, 224, 132
Bonnell I. A., Bate M. R., 2002, MNRAS, 336, 659
Bonnell I. A., Bate M. R., Zinnecker H., 1998, MNRAS, 298, 93
Briggs K. R., Pye J. P., 2003, MNRAS, 345, 714
Carpenter J. M., 2001, AJ, 121, 2851
Cutri R. M., Skrutskie M. F., Van Dyk S. et al., 2003, The 2MASS All-Sky Catalog of Point Sources, University of Massachusetts and Infrared Processing and Analysis Center (IPAC/California Institute of Technology)
Damiani F., Flaccomio E., Micela G., Sciortino S., Harnden F. R. Jr., Murray S. S., 2004, ApJ, 608, 781
Favata F., 2005, Mem. Soc. Astron. Ital., 76, 337
Feigelson E. D., Montmerle T., 1999, ARA&A, 37, 363
Raboud D., Cramer N., Bernasconi P. A., 1997, A&A, 325, 167
Rauw G., Nazé Y., Gosset E., Stevens I., Blomme R., Corcoran M. F., Pittard J. M., Runacres M. C., 2002, A&A, 395, 499
Rauw G., De Becker M., Gosset E., Pittard J. M., Stevens I. R., 2003, A&A, 407, 925
Rieke G. H., Lebofsky M. J., 1985, ApJ, 288, 618
Sana H., Stevens I. R., Gosset E., Rauw G., Vreux J.-M., 2004, MNRAS, 350, 809
Sana H., Gosset E., Rauw G., Sung H., Vreux J.-M., 2006a, A&A, 454, 1047 (Paper I)
Sana H., Rauw G., Nazé Y., Gosset E., Vreux J.-M., 2006b, MNRAS, 372, 661 (Paper II)
Schmidt-Kaler T., 1982, in Physical Parameters of the Stars, Landolt-Börnstein Numerical Data and Functional Relationships in Science and Technology, Group VI, Vol. 2b, Springer-Verlag, Berlin
