SOAR BVI photometry of the metal-poor bulge globular cluster NGC 6642

¹ Universidade de São Paulo, Dept. de Astronomia, Rua do Matão 1226, São Paulo 05508-090, Brazil e-mail: barbuy@astro.iag.usp.br
² Universidade Federal do Rio Grande do Sul, Dept. de Astronomia, CP 15051, Porto Alegre 91501-970, Brazil e-mail: [bica;charles]@if.ufrgs.br
³ Università di Padova, Dipartimento di Astronomia, Vicolo dell'Osservatorio 2, 35122 Padova, Italy e-mail: ortolani@pd.astro.it

Received: 19 August 2005
Accepted: 15 November 2005

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 for NGC 6642, and a distance from the Sun of  kpc. Therefore NGC 6642 is a moderately metal-poor globular cluster that is spatially located in the bulge at a galactocentric distance of  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.