UBVI CCD photometry of two old open clusters NGC 1798 and NGC 2192

Abstract

We present UBVI CCD photometry of two open clusters NGC 1798 and NGC 2192 which were little studied before. Color-magnitude diagrams of these clusters show several features typical for old open clusters: a well-defined main-sequence, a red giant clump, and a small number of red giants. The main sequence of NGC 1798 shows a distinct gap at V ~ 16.2 mag. From the surface number density distribution we have measured the size of the clusters, obtaining 8'.3 (= 10.2 pc) for NGC 1798 and 7'.3 (= 7.5 pc) for NGC 2192. Then we have determined the reddening, metallicity, and distance of these clusters using the color-color diagrams and color-magnitude diagrams: E(B-V)=0. 51\pm0.04, [Fe/H] = -0.47\pm0.15 dex and (m-M)_0=13.1\pm0.2 (d=4.2\pm 0.3 kpc) for NGC 1798, and E(B-V)=0.20\pm0.03, [Fe/H] = -0.31\pm0.15 dex and (m-M)_0=12.7\pm0.2 (d=3.5\pm 0.3 kpc) for NGC 2192. The ages of these clusters have been estimated using the morphological age indicators and the isochrone fitting with the Padova isochrones: 1.4 \pm 0.2 Gyrs for NGC 1798 and 1.1 \pm 0.1 Gyrs for NGC 2192. The luminosity functions of the main sequence stars in these clusters are found to be similar to other old open clusters. The metallicity and distance of these clusters are consistent with the relation between the metallicity and galactocentric distance of other old open clusters.