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G. De Marchi (ESA/STScI), F. Paresce (ESO), G. Andreuzzi, L. Pulone, R. Buonanno (Obs. Rome), F.R. Ferraro (Obs. Bologna)
We have carried out extensive VLT-FORS1 observations covering a fair fraction of the intermediate metallicity globular cluster NGC6712 in the V and R bands. We derive accurate colour-magnitude diagrammes and luminosity functions (LF) of the cluster main sequence (MS) for different fields extending from the centre of the cluster out to ~10' away, well beyond the nominal tidal radius and including a control field at ~42' distance. The LF extend from the cluster turn-off (TO) at MR ~4 to the point at which incompleteness drops below 50% at R ~ 23, corresponding to MR ~7.5 for most fields studied. Cluster stars become indistinguishable from field stars at r ~5'. The shape of the cluster's LF and its variation with distance from the centre in these ranges are well described by a standard multi-mass static model having the following parameters: core radius rc = 55", half-light radius rh = 1.8', tidal radius rt = 5.1', concentration ratio c=0.75, and a power-law global mass function (MF) with index alpha ~0.9 for masses smaller than 0.8 MO, i.e. for all detected MS stars. The MF obtained in this way is consistent with that found in a preliminary investigation of this cluster with the VLT Test Camera and confirms that this is the only globular cluster known so far for which the global MF drops with decreasing mass below the TO. Possible reasons for this unique characteristic are discussed with the most likely associated with its extreme vulnerability to tidal disruption.
The author(s) of this abstract have provided an email address for comments about the abstract: demarchi@stsci.edu