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2005A&A...438..291G - Astronomy and Astrophysics, volume 438, 291-300 (2005/7-4)

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


Abstract (from CDS):

I present the analysis results from XMM-Newton observations of the old open stellar cluster NGC 188, which has an age of about 7Gyr 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 1030erg/s. 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.

Abstract Copyright:

Journal keyword(s): open clusters and associations: general - stars: activity - stars: coronae - stars: evolution - X-rays: stars

VizieR on-line data: <Available at CDS (J/A+A/438/291): table1.dat>

Nomenclature: Table 1: [G2005] NN (Nos 1-58).

CDS comments: in ref list Binnendijk 1970VA.....12..217B instead of VA.12.211 , KALUZNY 1987ApJ...314..585K instead of AJ.314

Simbad objects: 63

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