Binary Star Evolution in Different Environments: Filamentary, Fractal, Halo, and Tidal Tail Clusters

Using membership of 85 open clusters from previous studies based on Gaia Data Release 3 data, we identify binary candidates in the color–magnitude diagram for systems with mass ratio q > 0.4. The binary fraction is corrected for incompleteness at different distances due to the Gaia angular resolution limit. We find a decreasing binary fraction with increasing cluster age, with substantial scatter. For clusters with a total mass >200 M⊙, the binary fraction is independent of cluster mass. The binary fraction depends strongly on stellar density. Among the four types of cluster environments, the lowest-density filamentary and fractal stellar groups have the highest mean binary fraction: 23.6% and 23.2%, respectively. The mean binary fraction in tidal tail clusters is 20.8% and is lowest in the densest halo-type clusters: 14.8%. We find clear evidence of early disruptions of binary stars in the cluster sample. The radial binary fraction depends strongly on the clustercentric distance across all four types of environments, with the smallest binary fraction within the half-mass radius rh and increasing toward a few rh. Only hints of mass segregation are found in the target clusters. The observed amounts of mass segregation are not significant enough to generate a global effect inside the target clusters. We evaluate the bias of unresolved binary systems (assuming a primary mass of 1 M⊙) in 1D tangential velocity, which is 0.1–1 km s−1. Further studies are required to characterize the internal star cluster kinematics using Gaia proper motions.