CCD photometry of the globular cluster NGC 6752

Abstract

A V, B−V colour–magnitude diagram for the globular cluster NGC6752 is presented. The use of a CCD has resulted in a colour–magnitude diagram with a very well-defined sequence going fainter than previously published work, and showing small scatter and little contamination. The distance to the cluster has been accurately tied to the trigonometrical parallaxes of local subdwarfs, giving a true distance modulus of 13.17±0.17 mag. The absolute magnitude of the horizontal branch, taken at the hypothetical centre of the instability strip, is found to be MV = +0.39±0.19. The age and metallicity of the cluster have been found from matching to the theoretical isochrones of VandenBerg & Bell. An age of 16±1 Gyr and a metallicity of [Fe/H] = −1.5±0.2 are derived, and the age spread of stars in the cluster must be less than 0.5 Gyr. The observed lower main sequence has a shallower slope than the theoretical isochrones. The spread in metallicity amongst the stars in the cluster is limited to ±0.15 dex by the small scatter on the subgiant branch. The luminosity function of the lower main sequence is similar to that of local Population I stars. No cluster white dwarfs are found.