Multicolor CCD photometry and stellar evolutionary analysis of NGC 1907, NGC 1912, NGC 2383, NGC 2384, and NGC 6709 using synthetic color-magnitude diagrams

Abstract

We present the first CCD photometric observations of NGC 2383 and NGC 2384 in B, V, R, and I, NGC 1912, NGC 6709 in B, V, and I and NGC 1907 in B and V passbands, reaching down to a limiting magnitude of V ~ 20 mag for ~3300 stars put together. The results of the spectroscopic observations of 43 bright stars in the field of NGC 1912, NGC 2383, NGC 2384, and NGC 6709 are also presented. The color-magnitude diagrams (CMDs) of the clusters in V versus B-V, V versus V-R, and V versus V-I are presented. The distances and reddening to these clusters are determined using the cluster CMDs. The distances to the clusters NGC 1907, NGC 1912, NGC 2383, NGC 2384, and NGC 6709 are 1785 ± 260, 1820 ± 265, 3340 ± 490, 2925 ± 430, and 1190 ± 175 pc, respectively. Some gaps in the cluster main sequence have been identified. We have compared the observed color-magnitude diagrams of these four open clusters with the synthetic ones derived from one classical and two overshoot stellar evolutionary models. Overshoot models estimate older ages for clusters when compared to the classical models. The age of the clusters estimated using the isochrones of Bertelli et al. are 400, 250, 400, 20, and 315 Myr for the clusters NGC 1907, NGC 1912, NGC 2383, NGC 2384, and NGC 6709, respectively. A comparison of the synthetic color-magnitude diagrams with the observed ones indicates that the overshoot models should be preferred. The comparison of integrated luminosity functions do not clearly indicate as to which model is to be preferred. The values of the mass function slopes estimated for the clusters are x = 1.7 ± 0.15 for NGC 1912 (mass range: 1.7–3.9 M_⊙) and NGC 6709 (1.7–3.4 M_⊙), x = 1.3 ± 0.15 for NGC 2383 (1.7–3.1 M_⊙), and x = 1.0 ± 0.15 for NGC 2384 (2.0–14.0). The present age estimates show that the closely located cluster pair NGC 1912 + NGC 1907 have similar ages, indicating that they may have born together, making them a good candidate to be a binary open cluster.