http://arxiv.org/abs/2112.08028
We propose a mixture model of open clusters (OCs) in the color-magnitude diagram (CMD) to measure the OC properties, including isochrone parameters (age, distance, metallicity, and dust extinction), stellar mass function, and binary parameters (binary fraction and mass ratio distribution), with unprecedented precision and reliability. The model treats an OC in the CMD as a mixture of single and binary member stars, and field stars in the same region. The cluster members are modeled based on the theoretical stellar model, mass function, and binary properties. The field component is modeled nonparametrically using a separate field star sample in the vicinity of the cluster. Unlike conventional methods that rely on stringent member selection, ours allows us to use a sample of more cluster members and attendant field stars. The larger star sample reduces the statistical error and minimizes the potential bias by retaining more stars, which is crucial for age estimation and mass function measurement. After testing with 1000 mock clusters, we measured the parameters of ten real OCs using Gaia EDR3 data. The best-fit isochrones are consistent with previous measurements in general, and provide more reasonable estimates of age for several OCs. Our method shows significantly better precision, 0.01 dex for logarithmic age and 0.01 mag for distance modulus. The inferred MF slope is -2.7 to -1.6 for clusters younger than 2 Gyr, while older clusters appear to have significantly flatter MFs. The binary fraction is 30 to 50%. The photometric and astrometric distances accord well.
L. Li and Z. Shao
Thu, 16 Dec 21
34/83
Comments: 17 pages, 8 figures, submitted to ApJ
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