The globular cluster BH 176 revisited

Abstract

Context. First classified as an open cluster, BH 176 lies in a crowded field at low galactic latitude. Later studies have labeled it a transition-type object, a metal-rich globular, or an open cluster, with distance estimates in the range of 15 to 85 kpc, indicating that the true nature of this cluster is rather uncertain. Aims. Our aim is to determine more precisely the distance to BH 176, and, as a matter of consequence, its metallicity and age. Methods. We have obtained two color-magnitude diagrams of BH 176, one in the infrared using the 2MASS point-source catalog, the other in the optical using FORS2 VLT archival images. We identified the red giant branch in the infrared and the red giant branch, red giant bump, red horizontal branch, subgiant branch, and main sequence turn-off in the optical, by selecting stars within ∼2-3′ from the center of the cluster. Results. We estimate a distance D ∼15.1 ± 0.5 kpc from the visual magnitude of the red clump and a metallicity [Fe/H] ∼-0.10 ± 0.1 dex from the magnitudes and colors of the main features in the two color-magnitude diagrams. Fitting isochrones from the Padova database to the color-magnitude diagrams confirms our results. Conclusions. BH 176 is an old metal-rich open or transition-type cluster, moderately close to the Galactic plane. It is a spectacular object for studying the early formation history of our Galaxy. © 2011 ESO.