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

T. Belloni·F. Verbunt·R. D. Mathieu
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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 orbital period > 700 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.

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T. Belloni, F. Verbunt, R. D. Mathieu. 1998-08-28. X rays from old open clusters: M 67 and NGC 188. https://arxiv.org/abs/astro-ph/9808329

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