EDP Sciences
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Volume 375, Number 1, August III 2001
Page(s) 30 - 39
Section Stellar clusters and associations
DOI http://dx.doi.org/10.1051/0004-6361:20010845

A&A 375, 30-39 (2001)
DOI: 10.1051/0004-6361:20010845

Red giants in open clusters

IX. NGC 2324, 2818, 3960 and 6259
J.-C. Mermilliod1, J. J. Clariá2, J. Andersen3, A. E. Piatti2 and M. Mayor4

1  Institut d'Astronomie de l'Université de Lausanne, 1290 Chavannes-des-Bois, Switzerland
2  Observatorio Astronómico, Laprida 854, 5000 Córdoba, Argentina
3  Niels Bohr Institute for Astronomy, Physics, and Geophysics, Astronomical Observatory, Juliane Maries Vej 30, 2100 Copenhagen, Denmark
4  Observatoire de Genève, 1290 Sauverny, Switzerland

(Received 20 February 2001 / Accepted 14 June 2001 )

We present accurate radial velocities and photoelectric UBV photometry for 73 and 57 red-giant candidates, respectively, in the intermediate-age open clusters NGC 2324, 2818, 3960 and 6259. These data confirm the membership of 47 stars, 12 of which (26% ) are spectroscopic binaries; three preliminary orbits have been determined in NGC 3960. From Washington photometry of 8 red giant members, the metallicity of NGC 6259 is found to be [ Fe/H $] = 0.06 \pm 0.08$. At the age of these clusters, most of the red giants are observed in the core-helium (clump) burning phase, the general morphology of which is well reproduced by theoretical models with convective overshooting. However, a number of bona fide cluster giant members are found significantly to the red of the isochrones fitting the rest of the CMD of these and a few other clusters. Some of these stars are binaries, but others seem to be single. In either case, their red colours and/or low luminosities remain unexplained by current stellar evolution theory.

Key words: cluster: open -- individual: NGC 2324, NGC 2818, NGC 3960, NGC 6259 -- binary: spectroscopic -- star: evolution -- star: red giant

Offprint request: J.-C. Mermilliod, Jean-Claude.Mermilliod@obs.unige.ch

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