A photometric and spectroscopic study of the southern open clusters Pismis 18, Pismis 19, NGC 6005, and NGC 6253

Abstract

CCD observations in the B, V , and I passbands have been used to generate color-magnitude diagrams (CMDs) for the southern open cluster candidates Pismis 18, Pismis 19, and NGC 6005, as well as for the old open cluster NGC 6253. The sample consists of about 1550 stars reaching down to V ~ 19 mag. From analysis of the CMDs, the physical reality of the three cluster candidates is conÐrmed and their reddening, distance, and age are derived, as well as those of NGC 6253. In addition, integrated spectra for Pismis 18, Pismis 19, and NGC 6253 covering a range from 3500 to 9200 Ŏ were obtained. The reddening, age, and metallicity of these three clusters were derived from Balmer and Ca II triplet equivalent widths by comparing the observed spectra with those of template clusters. The photometric and spectroscopic results allow us to conclude that Pismis 18, Pismis 19, and NGC 6005 are intermediate-age (t≈1 Gyr), genuine open clusters, while NGC 6253 has an age t≈5 Gyr. All these clusters are located approximately toward the Galactic center, between 1.5 and 2.7 kpc from the Sun. Their metallicities range over values that are typical of moderately metal-poor to moderately metal-rich open clusters.