A photometric and astrometric study of the open clusters NGC 1664 and NGC 6939

Abstract

This study calculated astrophysical parameters, as well as kinematic and galactic orbital parameters, of the open clusters NGC 1664 and NGC 6939. The work is based on CCD UBV and Gaia photometric and astrometric data from ground and space-based observations. Considering Gaia Early Data Release 3 (EDR3) astrometric data, we determined membership probabilities of stars located in both of the clusters. We used two-color diagrams to determine $E(B-V)$ color excesses for NGC 1664 and NGC 6939 as $0.190\pm 0.018$ and $0.380\pm 0.025$ mag, respectively. Photometric metallicities for the two clusters were estimated as [Fe/H] = $-0.10\pm 0.02$ dex for NGC 1664 and as [Fe/H] = $-0.06\pm 0.01$ dex for NGC 6939. Using the reddening and metallicity calculated in the study, we obtained distance moduli and ages of the clusters by fitting PARSEC isochrones to the color-magnitude diagrams based on the most likely member stars. Isochrone fitting distances are $1289\pm 47$ pc and $1716\pm 87$ pc, which coincide with ages of $675\pm 50$ Myr and $1.5\pm 0.2$ Gyr for NGC 1664 and NGC 6939, respectively. We also derived the distances to the clusters using Gaia trigonometric parallaxes and compared these estimates with the literature. We concluded that the results are in good agreement with those given by the current study. Present day mass function slopes were calculated as $\Gamma =-1.22\pm 0.33$ and $\Gamma =-1.18\pm 0.21$ for NGC 1664 and NGC 6939, respectively, which are compatible with the Salpeter (1955) slope. Analyses showed that both of clusters are dynamically relaxed. The kinematic and dynamic orbital parameters of the clusters were calculated, indicating that the birthplaces of the clusters are outside the solar circle.

44 pages, including 16 figures and 9 tables, accepted for publication in the Astronomical Journal

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