A study of the old galactic star cluster Berkeley 32

Abstract

We present new CCD photometry of the distant old open star cluster Berkeley 32 in Johnson V and Cousins I passbands. A total of ~ 3200 stars have been observed in a field of about 13' × 13'. The colour-magnitude diagram (CMD) in V, (V - I) has been generated down to V = 22 mag. A broad but well defined main sequence is clearly visible. Some blue stragglers, a well developed subgiant branch and a Red Clump are also seen. By fitting isochrones to this CMD as well as to other CMDs available in the literature, and using the Red Clump location, the reddening, distance and age of the star cluster have been determined. The cluster has a distance of ~ 3.3 kpc, its radius is about 2.4 pc; the reddening E(B-V) is 0.08 mag and the age is ~ 6.3 Gyr. By comparison with theoretical isochrones, a metallicity of [Fe/H] ≈-0.2 dex has been estimated. Theoretical isochrones have been used to convert the observed cluster luminosity function into a mass function in the mass range ~ 0.6-1.1 M_⊙. We find a much flatter mass function than what has been found for young clusters. If the mass function is a power law dN ~ m^αdm, then we get α= -0.5 ± 0.3. This may be seen as a signature of the highly evolved dynamical state of the cluster.