1 Introduction

NGC 6712 ( $\alpha = 18^{\rm h}53^{\rm m} 04.3''$, $\delta = -08^{\rm o}42'21.5''$) is a small and sparse globular cluster of intermediate metallicity (concentration ratio c=0.9 and [Fe/H]=-1.01; Harris 1997). Cudworth (1988), in his astrometric and photometric study reaching down to just above the main sequence TO, finds that it is a halo object in spite of its moderately high metallicity. One interesting characteristic of this cluster is the presence in the core of the high luminosity X-ray source (X1850-086) with an optical counterpart (Anderson et al. 1993). This is unexpected for such a loose cluster because most clusters with such sources tend to have a much higher central concentration. To explain this peculiarity, Grindlay et al. (1988) have suggested that the cluster may be currently re-expanding following the phase of core collapse when densities were high enough to allow these binaries to be formed. Our discovery (Ferraro et al. 2000) of the presence in the core of another close binary, a UV- and $H\alpha$-excess object, most likely a quiescent LMXB or a CV, only adds to the mystery.

Another, possibly connected and potentially even more interesting facet of this cluster's structure is the fact that the first observations of its MS taken by the VLT during its commissioning period and reported by De Marchi et al. (1999) show a remarkable property of its MF near the half-light radius. This is a clear and continuous drop with decreasing mass starting already at the TO and continuing down to the observation limit at $\sim$0.5 $M_{\odot }$. MFs determined from LFs obtained near the half-light radius are expected to faithfully reflect the shape of the cluster's global MF (De Marchi et al. 2000; Vesperini & Heggie 1997). For all the other clusters surveyed so far with HST in this mass range, the global MF increases steadily with decreasing mass (Paresce & De Marchi 2000).

As suggested by De Marchi et al. (1999), this may be due to the fact that its Galactic orbit forces the cluster to penetrate deeply into the bulge. With a perigalactic distance smaller than 300pc, this cluster ventures so frequently and so deeply into the Galactic bulge (Dauphole et al. 1996) that it is likely to have undergone severe tidal shocking during the numerous encounters with both the disk and the bulge during its life-time. The latest Galactic plane crossing could have happened as recently as $4\times 10^6$ year ago (Cudworth 1988), which is much smaller than its half-mass relaxation time of 1Gyr (Harris 1996). It is precisely on this basis that Takahashi & Portegies Zwart (2000) have suggested that NGC 6712 has lost 99% of its mass during its life-time. And if the effects of this strong interaction have propagated throughout the whole cluster and reached its innermost regions, as such a gigantic mass loss implies, the peculiarly high density of core binaries can be understood and justified for what would otherwise appear an inconsequential cluster.

In an attempt to clarify this important issue and to better understand the observable effects of tidal interactions, and especially to learn more about the mechanisms leading to the dissolution of globular clusters in the Galaxy and about the possible variation of the cluster IMF with time in general, we have used the great power of the VLT and the FORS1 camera to investigate in more detail the present structure of NGC 6712. The specific objective was to obtain a more precise LF of the MS below the TO at various distances from the centre, so as to evaluate the possible effects of mass segregation on the derived MF. Another important objective was to sample more of the cluster at or near the tidal radius to see whether or not one could detect an excess of low mass stars ejected from the interior and still lightly bound to the cluster and to correct for the numerous field stars expected in the cluster field. In this paper, we report on the results of the analysis of our VLT data set on NGC 6712 with an emphasis on the cluster MS. The analysis of the evolved part of the cluster CMD is the subject of other papers (Ferraro et al. 2000; Paltrinieri et al. 2001).