VLT observations of the peculiar globular cluster NGC 6712 - II. luminosity and mass functions

We have carried out extensive VLT-FORS1 observations covering a fair fraction of the intermediate metallicity globular cluster NGC 6712 in the V and R bands. We derive accurate colour-magnitude diagrammes (CMD) and luminosity functions (LFs) of the cluster main sequence (MS) for four overlapping fields extending from the centre of the cluster out to a radius of similar to 10', well beyond the nominal tidal radius, and for a control field at similar to 42' distance. The LFs extend from the cluster turn-off (TO) at M-R similar or equal to 4 to the point at which the incompleteness drops below 50% (corresponding to R similar or equal to 23 or M-R similar or equal to 7.5) for most fields studied. Cluster stars become indistinguishable from field stars at r similar or equal to 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 r(c) = 1', half-light radius r(h1) = 1.'8, tidal radius r(t) = 5.'2, concentration ratio c = 0.7, and a power-law global mass function (MF) with index alpha similar or equal to 0.9 for masses smaller than 0.8 M-circle dot, i.e. for all detected MS stars, and alpha similar or equal to -2.35 for evolved objects. 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.