Binaries in the Praesepe and Coma Star Clusters and Their Implications for Binary Evolution

Abstract

This completes a study of the evolution of binary systems in five open clusters of various ages. Among 21 stars observed in Praesepe, eight are found or confirmed to be spectroscopic binaries and orbital elements are derived, while one more shows long-term binary motion. Among 18 stars observed in the Coma Berenices cluster, five are found or confirmed to be spectroscopic binaries and orbital elements are derived, while a sixth has tentative elements. Among five clusters studied we searched for three expected evolutionary effects, namely an increase with age in the mass ratios, a decrease with age of the binary periods, and an increase in binary frequencies. We find that there is a progression (at the 3 σ level) from no binaries out of 10 with mass ratios greater than 0.5 in the youngest cluster (combined with the published results for NGC 6193) to 25% such stars in the intermediate-age clusters to 43% such stars in these two oldest clusters. There is no evidence for an increase in short-period binaries with age. And there is slight evidence (at the 1 σ level) for an increase with age from 15% to 28% in the fraction of large-amplitude binaries. These results are mostly consistent with the idea that most binaries are formed or modified in three-body interactions, and successive generations of formation and disruptions tend to form binaries with larger mass ratios. However, part of the initial generation of binaries is probably primordial.