343 – 343 - Astrostatistics Interest Group: Student Paper Award
Dynamical Interaction in the Stellar Cluster: Evidence from Binaries of NGC3532
Lu Li
Shanghai Astronomical Observatory, CAS
The binary properties of open clusters impose important restrictions on star formation theories and clusters' dynamical evolution. We establish a comprehensive method for modeling the cluster members' color-magnitude diagram (CMD) as a mix of single stars and photometric unresolved binaries. Using this method, we can infer the binary fraction f_b and the binary mass-ratio distribution index ?_q when a power-law is assumed, with high precision and accuracy. We employ a modified Gaussian process to determine the main sequence ridgeline and its scatter from the observed CMD as model input. We apply the method to the open cluster NGC3532 with the Gaia DR2 photometry. For the cluster members within a magnitude range corresponding to FGK dwarfs, we obtain f_b=0.267±0.019 and ?_q =-0.10±0.22 for binaries with mass ratio q>0.2. The f_b value is consistent with the previous work on NGC3532 and smaller than that of field stars. The near-zero ?_q implies a nearly uniform distribution of binary mass ratios. For the first time, we unveil that the stars with smaller mass or in the inner region tend to have lower f_b and more positive value of ?_q due to the lack of low mass-ratio binaries. The clear dependences of binary properties on mass and radius are most likely caused by the internal dynamics.