The intermediate-age open cluster NGC 2158

Abstract

We report on UBVRI CCD photometry of two overlapping fields in the region of the intermediate-age open cluster NGC 2158 down to V = 21. By analysing colour-colour (CC) and colour-magnitude diagrams (CMDs) we infer a reddening E(B - V) = 0.55 ± 0.10, a distance of 3600 ± 400 pc, and an age of about 2 Gyr. Synthetic CMDs constructed with these parameters (but fixing E(B - V) = 0.60 and [Fe/H] = -0.60), and including binaries, field contamination and photometric errors, yield a good description of the observed CMD. The elongated shape of the clump of red giants in the CMD is interpreted as resulting from a differential reddening of about ΔE(B - V) = 0.06 across the cluster, in the direction perpendicular to the Galactic plane. NGC 2158 turns out to be an intermediate-age open cluster with an anomalously low metal content. The combination of these parameters, together with the analysis of the cluster orbit, suggests that the cluster belongs to the old thin disc population.