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Astron. Astrophys. 339, 431-439 (1998)

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X-rays from old open clusters: M 67 and NGC 188

T. Belloni 1, F. Verbunt 2 and R.D. Mathieu 3

1 Astronomical Institute "Anton Pannekoek", University of Amsterdam and Center for High-Energy Astrophysics, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
2 Astronomical Institute, P.O.Box 80000, 3508 TA Utrecht, The Netherlands
3 Department of Astronomy, University of Wisconsin, Madison, WI 53706, USA

Received 15 July 1998 / Accepted 17 August 1998

Abstract

We have observed the old open clusters M 67 and NGC 188 with the ROSAT PSPC. In M 67 we detect a variety of X-ray sources. The X-ray emission by a cataclysmic variable, a single hot white dwarf, two contact binaries, and some RS CVn systems is as expected. The X-ray emission by two binaries located below the subgiant branch in the Hertzsprung Russell diagram of the cluster, by a circular binary with a cool white dwarf, and by two eccentric binaries with [FORMULA]d is puzzling. Two members of NGC 188 are detected, including the FK Com type star D719. Another possible FK Com type star, probably not a member of NGC 188, is also detected.

Key words: stars: abundances – open clusters: individual: M 67; NGC 188 – X-rays: stars

Send offprint requests to: T. Belloni

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© European Southern Observatory (ESO) 1998

Online publication: October 21, 1998
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