Red giants in open clusters

Abstract

New, accurate radial velocities and photoelectric ${\it UBV}$ photometry of 83 red-giant candidates in the field of the rich, intermediate-age open cluster NGC 2477 ($\rm [Fe/H]= -0.05$, $\rm age \simeq1$ Gyr) are presented and discussed. From 49 constant-velocity members we find a mean cluster velocity of $+7.32\pm0.13$ km s -1 and confirm the membership of 76 of the stars. Among the cluster members, we identify 26 definite and 1 probable spectroscopic binaries and determine orbits for 13 of these systems, with periods ranging from 40 to 4578 days. The binary frequency is thus rather high ($27/76 = 36\%$). The observed internal radial velocity dispersion of the cluster, as determined from the single member stars, is 0.93 km s -1 , corrected for the small average observational error of 0.22 km s -1 . Fitting King-type models to the observed stellar density distribution and velocity dispersion, and assuming a distance of 1.25 kpc, we find the core and tidal radii of NGC 2477 to be 1.8 and 8.1 pc, respectively, and estimate that the mass of cluster stars down to $V = 17$, corresponding to ~$1~M_{\odot}$, is at least 5400 $M_{\odot}$. The substantial differential reddening of NGC 2477 requires a more detailed study before definitive isochrone fits can be made.