Red giants in open clusters

Abstract

Aims. We studied the membership and binarity of 123 red giants in six old open clusters, NGC 2112, 2204, 2243, 2420, 2506 and 2682, to define more precisely the evolutionary path on the red-giant branch. Methods. The analysis is based on 185 radial-velocity observations with the Coravel spectrographs and available photometric data. Results. The membership of 93 red giants was confirmed on the basis of the radial velocities. Seven definitive spectroscopic binaries were identified and 11 additional stars are suspected of being binaries. The binary frequency (19%) is slightly lower than average. This is partly due to the small number of observations secured for each star. Orbital elements have been determined for the first time for the BaII star NGC 2420-173 (D) and those of the other BaII star NGC 2420-250 (X) have been improved. The values of the cluster mean velocities have been significantly improved. Conclusions. With the new membership estimates and binary detections, the existing CCD data allow precise definition of the redgiant loci. A number of stars in NGC 2506, 2420 and 2204 appear to define an asymptotic branch, the position of which differs significantly from that predicted by the models.