Red giants in open clusters

Abstract

Context. The identification and characterisation of spectroscopic binaries with red-giant primaries in open clusters is important for a proper understanding of the colour-magnitude diagrams of the clusters. Moreover, the orbital eccentricities and axial rotations of these binaries are valuable probes into the inner structure and tidal interaction of the stars. Aims. We report on a comprehensive, long-term monitoring programme aiming to improve our knowledge of such binary systems. Methods. The radial velocities of 1309 red giants in 187 open clusters in the whole sky have been monitored with the Coravel and CfA spectrometers for 20 years, with a typical accuracy of 0.4 km s −1 per observation. Results. In total, 289 spectroscopic binaries were detected in the sample. We present first orbits for 67 systems and improved elements for another 64 previously published orbits, based on additional observations. For completeness, 25 published orbits are listed as well. The orbits are based on a total of 4039 observations, an average of 26 per system. Orbital periods range from 41.5 to 14 722 days (40 yrs), eccentricities from 0.00 to 0.81. The remaining 133 systems have too long periods, too few observations, and/or inadequate phase coverage for an orbit determination at this time. Conclusions. This paper provides a dramatic increase in the body of homogeneous orbital data available for red-giant spectroscopic binaries in open clusters. It will form the basis for a comprehensive discussion of membership, kinematics, and stellar and tidal evolution in the parent clusters.