Astron. Astrophys. 343, L9-L14 (1999)

1. Introduction

NGC 6712 is a small (tidal radius ) and relatively loose () and faint (; Djorgovski 1993) Galactic globular cluster (GC) that has not yet received much observational attention. Its main claim to fame so far is due to the presence in its core of the high luminosity X-ray burster X1850-086 whose optical counterpart may be a faint UV-excess object (Anderson et al. 1993). This fact presents somewhat of a puzzle since one would expect such an X-ray source to be located in a highly concentrated cluster where the stellar density favors its formation via tidal capture of a neutron star (Hertz & Grindlay 1985). Most other sources of this type have indeed been found in high density core collapse clusters suggesting that, perhaps, NGC 6712 has already undergone such an event in the past and is now in a state of re-expansion (Grindlay et al. 1988).

This unusual situation may also be connected in some way to its Galactic orbit as computed recently by Dauphole et al.(1996) that is fairly well restricted to the vicinity of the disk and penetrates very deeply in the Galactic bulge. This certainly means that one would expect this cluster to have undergone severe tidal shocking during the numerous encounters with both the disk and the bulge during its lifetime and the consequences on the dynamical status of the cluster to be significant and observable. A simple single-mass approximation of these effects was computed by Gnedin & Ostriker (1997) for both disk and bulge shocks under differing assumptions on the Galactic model with a resultant time to destruction as small as 0.03 H. According to these calculations, then, the cluster should have evaporated long ago and at the very least may have lost a very substantial portion of its original mass during its lifetime.

Clearly, this catastrophe should be well impressed on its present day distribution of stars on the main sequence (MS) with its lowest mass members beyond the half-mass radius particularly vulnerable to escape. This effect may well have been detected already in M 4, another cluster at significant risk of tidal disruption (Kanatas et al. 1994; Pulone et al. 1998a), but until this cluster's structural parameters are pinned down more accurately this remains still speculative. There is, therefore, much interest today in determining accurately the present day mass function (PDMF) of NGC 6712 to look for the signature of such powerful effects. Currently available observations of the color-magnitude diagram (CMD) of this cluster, however, only reach to just above the MS turn-off (Cudworth 1988; Anderson et al. 1993) and are, therefore, of limited use for this task. In order to push the observations well into the relevant part of the MS below the turn-off (TO), the VLT was used to probe deeply into this cluster with its unprecedented sensitivity and resolution. This paper describes the first results of these observations that give clear evidence that there is indeed a distortion of the MF of NGC 6712 with respect to that of other dynamically much less disturbed clusters.

© European Southern Observatory (ESO) 1999

Online publication: March 1, 1999
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