Red giants in open clusters*
IX. NGC 2324, 2818, 3960 and 6259
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
Corresponding author: J.-C. Mermilliod, Jean-Claude.Mermilliod@obs.unige.ch
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. 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: galaxy: open clusters and associations: individual: NGC 2324, NGC 2818, NGC 3960, NGC 6259 / stars: binaries: spectroscopic / stars: evolution / stars: AGB and post-AGB
Based on observations collected with the Danish 1.54-m telescope at the European Southern Observatory, La Silla (Chile) and at Cerro Tololo Inter-American Observatory, National Optical Astronomy Observatories, operated by the Association of the Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.
© ESO, 2001