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

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

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

1</sup> 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 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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Contents

• 1. Introduction
• 2. Observations and data analysis
  • 2.1. M 67
    • 2.1.1. X-ray data
    • 2.1.2. Optical identifications
  • 2.2. NGC 188
• 3. Results
  • 3.1. M 67
    • 3.1.1. The cataclysmic variable EU Cnc
    • 3.1.2. The hot white dwarf
    • 3.1.3. Contact binaries
    • 3.1.4. RS CVn systems
    • 3.1.5. S 1040
    • 3.1.6. Stars below the subgiant branch
    • 3.1.7. Other eccentric binaries
    • 3.1.8. No indication of binarity
    • 3.1.9. Non-members
  • 3.2. NGC 188 and V 8
• 4. Conclusions
• Acknowledgements
• References

© European Southern Observatory (ESO) 1998

Online publication: October 21, 1998
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