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

Abstract

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 r h and increasing toward a few r h. 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.

The Present-day Mass Function of Star Clusters in the Solar Neighborhood

“…The stellar mass, m 0 , obtained in the manner described above is determined under the assumption that observed stars are single stars. However, as shown by Pang et al (2023), the unresolved binary fraction (for mass ratio q > 0.4) among 85 star clusters ranges from 5% up to 40% (Figure 2(a) in Pang et al 2023). Note that these values are lower limits of the total unresolved binary fraction.…”

“…The uncertainty in the stellar mass m 0 is primarily determined by the photometric uncertainty of Gaia Data Release 3 (DR3; Gaia Collaboration et al 2023), which may result in variations in the locations of cluster members in the CMD, and variations in the estimation of the binary properties (Pang et al 2023). To determine the impact of these photometric uncertainties, we generate simulated observations.…”