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Astron. Astrophys. 337, 372-378 (1998)

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ROSAT HRI observations of the intermediate-age open cluster IC 4756

S. Randich 1, K.P. Singh 2, T. Simon 3, S.A. Drake 4 and J.H.M.M. Schmitt 5

1 Osservatorio Astrofisico di Arcetri, Largo Fermi 5, I-50125 Firenze, Italy
2 Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai 400 005, India
3 Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, Hawaii 96822, USA
4 Laboratory for High Energy Astrophysics, Goddard Space Flight Center, Greenbelt MD 20771, USA
5 Universität Hamburg, Hamburger Sternwarte, Gojenbergsweg 112, D-21029 Hamburg, Germany

Received 11 May 1998 / Accepted 4 June 1998


We have obtained an 88 kilosecond ROSAT HRI exposure of the intermediate-age open cluster IC 4756 with the purpose of detecting stars in the high luminosity tail ([FORMULA] erg s-1) of its X-ray luminosity distribution. However, only 1 cluster member (HSS 201) out of the 60 members inside the central high-sensitivity region of the HRI field of view (FOV) was detected. This star has spectral type A8, suggesting a close binary system with a low mass X-ray emitting companion. We compare the distribution of upper limits for F and G-type dwarfs in IC 4756 with the X-ray distribution functions of the similarly aged Hyades and Praesepe clusters. The results of this statistical analysis are inconclusive for G-type stars, but suggest that at least F-type stars in IC 4756 are not as X-ray luminous as their Hyades counterparts, thus indicating intrinsic differences between the two clusters. Finally, our data indicate a deficit of very active binaries with respect to both Hyades and Praesepe, and older open clusters.

Key words: stars: activity – X-rays: stars – open clusters and associations: individual: IC 4756

Send offprint requests to: S. Randich
Correspondence to: randich@arcetri.astro.it

© European Southern Observatory (ESO) 1998

Online publication: August 17, 1998