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Astron. Astrophys. 335, 517-521 (1998)

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Soft X-ray emission from intermediate-age open clusters: IC 4651

T. Belloni 1 and G. Tagliaferri 2

1 Astronomical Institute "Anton Pannekoek", University of Amsterdam and Center for High-Energy Astrophysics, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
2 Osservatorio Astronomico di Brera, Via Bianchi 46, I-22055 Merate, Italy

Received 28 October 1997 / Accepted 8 April 1998


We present the results of soft X-ray observations of the intermediate-age open cluster IC 4651 performed with the ROSAT PSPC. We detected 25 sources. Two are identified with a giant binary and a blue straggler respectively, both belonging to the cluster, and two with probable main-sequence members. Two other cases have ambiguous identification. Of the five binaries known in the cluster, the one detected in X rays is the only one whose period is short enough to maintain fast rotation and therefore strong stellar activity at a high age. The detected blue straggler is probably binary, suggesting that binarity is the key to producing a high level of X-ray emission. It is the third blue straggler detected in X rays. The remaining sources need to be identified through optical follow-up.

Key words: stars: activity – open clusters and associations: individual: IC 4651 – X-rays: stars

Send offprint requests to: T. Belloni

© European Southern Observatory (ESO) 1998

Online publication: June 18, 1998