Red giants in open clusters. III. Binarity and stelllar evolution in five intermediate-age clusters: NGC 2360, 2423, 5822, 6811 and IC 4756.

Coravel radial velocities of 93 red giants in five open clusters older than the Hyades (NGC 2360, 2423, 5822, 6811 and IC 4756) are analyzed for membership and binary detection. Seventeen binaries have been discovered among the 71 members, resulting in a binary percentage of 27%. One new orbit has been determined in IC 4756, with a long period (2000 days) and a very small eccentricity (e =0.04).

The cluster ages are around 2.2 10⁹ yr (9.3<logt<9.4), and the masses at the end of the main sequence are between 1.8 and 1.9 M_sun. The colour-magnitude diagrams of the red giants, displaying clumps and not long giant branches, imply that the stars do not evolve through helium flash. Therefore the limiting initial stellar mass for helium ignition in non-degenerate cores must be smaller than the standard value of 2.2 M0_sun The isochrone fitted in the diagram of NGC 2360 shows that the evolutionary models with core overshooting reproduce the observations better than the standard models (without overshooting) do, and that the structure of the red giant track can be described observationally. This also confirms the general morphology already found in the five Hyades-generation clusters and supports the previous detection of a concentration of stars about 0.2 to 0.4 mag below the clump.