SOARBVIphotometry of the metal-poor bulge globular cluster NGC 6642

Abstract

We present BVI photometry of the globular cluster NGC 6642 using the SOI imager at the SOAR Telescope. The colour magnitude diagrams (CMD) reach ≈ 1.5 mag in V below the main sequence turn-off. A comparison of the overall sequences, and in particular the Red Giant Branch slope of NGC 6642 with that of M 5, indicates that the two clusters must have a similar metallicity of [Fe/H] ≈ –1.3. We also obtained a reddening $E(B-V)=0.42\pm0.03$ for NGC 6642, and a distance from the Sun of $d_{\odot}=7.2\pm0.5$ kpc. Therefore NGC 6642 is a moderately metal-poor globular cluster that is spatially located in the bulge at a galactocentric distance of $R_{\rm GC}\approx 1.7$ kpc. The comparison of CMDs of NGC 6642 with those of M 5 shows that there is a very good match of magnitude difference between turn-off and horizontal branch, suggesting comparable ages. The age of M 5 is typical of halo globulars, so NGC 6642 is coeval with the halo. It is a good candidate as one of the few genuine metal-poor and old bulge clusters, and might be one of the oldest fossils in the Galaxy.