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Issue A&A
Volume 438, Number 1, July IV 2005
Page(s) 291 - 300
Section Stellar atmospheres
DOI http://dx.doi.org/10.1051/0004-6361:20052732



A&A 438, 291-300 (2005)
DOI: 10.1051/0004-6361:20052732

X-ray observations of the old open stellar cluster NGC 188

P. Gondoin

European Space Agency, ESTEC - Postbus 299, 2200 AG Noordwijk, The Netherlands
    e-mail: pgondoin@rssd.esa.int



(Received 20 January 2005 / Accepted 21 February 2005 )

Abstract
I present the analysis results from XMM-Newton observations of the old open stellar cluster NGC 188, which has an age of about 7 Gyr and a near solar metallicity. 58 X-ray sources were detected in the field of view of the EPIC MOS and pn cameras, and 46 sources are new X-ray detections. Visible counterparts were found for 20 sources including the variable star WV 28, the W UMa-type binaries V371 Cep and V372 Cep, and the red giant V11. 9 X-ray sources are identified with probable cluster non-members, while 43 X-ray sources are of unknown membership. X-ray emission was detected from 6 stars with high membership probability above a luminosity threshold of 1030 erg s-1. This indicates the presence of very active late-type stars in NGC 188 in spite of its old age. The HR diagram positions of two of these stars just above the main sequence are reminiscent of those for W Ursae Majoris-type contact binaries. Two other sources could be either members of close binary systems or the product of the coalescence of W UMa type binaries into single stars. One X-ray source in NGC 188 is located at the bottom of the red giant branch in an evolutionary status similar to that of an FK Comae-type star. Another X-ray source detected in NGC 188 has the HR diagram position of an M type star. Its X-ray to bolometric luminosity ratio, greater than the canonical 10-3 saturation level, suggests that the star was flaring during XMM-Newton observations. M stars are most likely the most numerous X-ray sources in NGC 188 at lower X-ray luminosity thresholds.


Key words: open clusters and associations: general -- stars: activity -- stars: coronae -- stars: evolution -- X-rays: stars

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© ESO 2005


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