Stellar Dynamics and Evolution of the Intermediate-Age Open Clusters NGC 2266 and NGC 2324

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Original abstract

We present a refined astrometric and photometric analysis of the well-studied intermediate-age open clusters NGC 2266 and NGC 2324 using high-precision Gaia DR3 data, complemented by 2MASS and LAMOST DR7 catalogs. Probable cluster members are identified using unsupervised machine learning techniques. We apply both Gaussian Mixture Models (GMMs) and \texttt{pyUPMASK}. We find that the GMM-based membership sample yields a cleaner, more coherent cluster sequence in the Gaia CMDs than pyUPMASK. We identified 719 and 852 high-probability members ($P \geq 0.7$) for NGC 2266 and NGC 2324, respectively. Using the parallax method, we determine distances of 3.55 $\pm$ 0.23~kpc for NGC 2266 and 4.18 $\pm$ 0.24~kpc for NGC 2324. The radius estimates for both clusters are 7.23 $\pm$ 0.47 pc and 10.94 $\pm$ 0.63 pc. Isochrone fitting estimated ages of $1.1 \pm 0.1$~Gyr for NGC 2266 and $790 \pm 150$~Myr for NGC 2324. These age estimates were derived assuming metallicities of $Z = 0.0084$ and $Z = 0.0038$, respectively. The King profile fitting indicates that both clusters exhibit compact, well-defined radial structures. Their tidal radii are $8.84'$ (9.13 pc) for NGC 2266 and $10.97'$ (13.34 pc) for NGC 2324. The slopes of the present-day mass functions are $1.13\pm0.18$ for NGC 2266 and $1.24\pm0.19$ for NGC 2324, indicating a deficiency of low-mass stars. The derived mass-function slopes are consistent with dynamical evolution in both clusters. The clusters exhibit short relaxation times, while only NGC 2324 shows a mild indication of mass segregation. This study highlights the power of Gaia astrometry to resolve internal structures within open clusters and refine their dynamical parameters.

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