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
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.
Key words: globular clusters: individual: NGC 6642 / Hertzsprung-Russell (HR) and C-M diagrams
© ESO, 2006