HST observations of the metal rich globular clusters NGC6496 and NGC6352

Abstract

Deep exposures of the metal-rich globular clusters NGC6496 and NGC6352 were obtained with the WFPC2 camera on board the HST through the F606W and F814W filters. The resulting colour-magnitude diagrams reach down to absolute magnitude M_814~10-10.5, approximately 5 magnitudes below the main sequence (MS) turn-off. The MS of the two clusters are sharp and well defined and their fiducial lines overlap almost exactly throughout this range. Their colour is, however, more than 0.1 mag redder than the MS fiducial line of the prototype metal-rich globular cluster 47 Tuc, after proper correction for the relative distances and reddening. This provides solid empirical evidence of a higher metal content, which is not surprising if these objects belong indeed to the bulge as their present location suggests. A good fit to the upper part of the MS of both clusters is obtained with a 10 Gyr-old theoretical isochrone from Baraffe et al. (1998) for a metallicity of [M/H]=-0.5, but at lower luminosity all models depart considerably from the observations, probably because of a deficiency in the treatment of the TiO opacity. The luminosity functions obtained from the observed CMD are rather similar to one another and show a peak at M_814~9. The present day mass functions (PDMF) are derived down to m~0.2 Msolar and are consistent with power-law indices alpha=0.7 for NGC6496 and alpha=0.6 for NGC6352. The PDMF of 47 Tuc is twice as steep in the same mass range (alpha=1.4). We investigate the origin of this discrepancy and show that it can be understood if the two clusters contain a considerably higher fraction of primordial binaries amongst their MS population, similar to that expected in the bulge. We briefly discuss the implications of this finding on the process of star and binary formation and on the universality of the IMF.