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

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

Jul 2005

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005a%26a...438..291g&link_type=abstract

Astronomy and Astrophysics, Volume 438, Issue 1, July IV 2005, pp.291-300

Astronomy and Astrophysics

Astrophysics

7

Open Clusters And Associations: General, Stars: Activity, Stars: Coronae, Stars: Evolution, X-Rays: Stars

Scientific paper

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.

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