VLT observations of the peculiar globular cluster NGC 6712^*

II. luminosity and mass functions

¹ Osservatorio Astronomico di Roma, via di Frascati 33, 00040 Monteporzio Catone, Rome, Italy
² European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany
³ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁴ Affiliated with the Astrophysics Division, Space Science Department, European Space Agency
⁵ Osservatorio Astronomico di Bologna, Via Ranzani 1, 40127, Bologna, Italy

Corresponding author: G. De Marchi, demarchi@stsci.edu

Received: 20 September 2000
Accepted: 13 March 2001

Abstract

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 ~, well beyond the nominal tidal radius, and for a control field at ~ distance. The LFs extend from the cluster turn-off (TO) at to the point at which the incompleteness drops below 50% (corresponding to or ) for most fields studied. Cluster stars become indistinguishable from field stars at . 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 , half-light radius , tidal radius , concentration ratio , and a power-law global mass function (MF) with index for masses smaller than 0.8 , i.e. for all detected MS stars, and 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.