Photometric and integrated spectral study of the young open clusters Pismis 22, NGC 6178, NGC 6216 and Ruprecht 130

Abstract

We present CCD observations in the B,V and I passbands obtained for stars in the fields of the open clusters Pismis 22, NGC 6178, NGC 6216 and Ruprecht 130, projected not far from the Galactic centre. The sample consists of about 790 stars reaching down to V = 18-19 mag. From the analysis of the colour-magnitude diagrams, we confirmed the physical reality of the clusters and derived their reddening, distance and age. In addition, we obtained flux-calibrated integrated spectra in the range 3500-9200 A for the cluster sample. The equivalent widths of the Balmer lines provided us with age estimates, while the comparison with template spectra allowed us to derive both foreground reddening and age. The photometric and spectroscopic results reveal that the four studied objects are young open clusters with ages ranging between 35 and 50 Myr. The clusters, located between 1.0 and 4.3 kpc from the Sun, are ffected by different amounts of interstellar visual absorption (0.6 < Av < 6.0).