UBVI CCD photometry and star counts in nine inner disc Galactic star clusters

Abstract

We present and discuss new CCD-based photometric material in the UBVI passbands for nine Galactic star clusters located inside the solar ring, for which no CCD data are currently available. These star clusters are IC 2714, NGC 4052, ESO 131SC09, NGC 5284, NGC 5316, NGC 5715, VdB-Hagen 164, NGC 6268 and Czernik 38. The main aim of this study is to establish the nature of real clusters or random field star enhancements and, when real, estimate their fundamental parameters. To this aim, we first perform star counts by combining our optical photometry with 2MASS, and derive cluster sizes and radial density profiles. The fundamental parameters such as age, reddening and distance are then inferred from the analysis of the star distribution in the colour-colour and colour-magnitude diagrams of only the spatially selected likely members. Our analysis shows that ESO 131SC09, NGC 5284 and VdB-Hagen 164 are most probably not clusters, but random enhancements of a few bright stars along the line of sight, with properties much similar to so-called open cluster remnants. The remaining clusters are physical groups and are all younger than about 1 Gyr. We use the newly derived set of parameters, in particular distance and reddening, to investigate their position in the Galaxy in the context of the spiral structure of the Milky Way. We find that the youngest clusters (IC 2714, NGC 5316 and NGC 6268) are located close to or inside the Carina-Sagittarius arm, and are therefore bona fide spiral structure tracers. On the other hand, the oldest clusters (Czernik 38, NGC 4052 and NGC 5715) are floating in the interarm space between the Carina-Sagittarius and the more distant Scutum-Crux arm. Interestingly enough, the oldest clusters of this sample - Czernik 38 and NGC 5715 - are among the few known open clusters to be older or as old as the Hyades in the inner Galactic disc, where star clusters are not expected to survive for a long time, because of the strong tidal field and the higher probability of close encounters. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.