Why is the mass function of NGC 6218 flat?

We have used the FORS-1 camera on the VLT to study the main sequence ( MS) of the globular cluster NGC 6218 in the V and R bands. The observations cover an area of 3.'4 x 3.'4 around the cluster centre and probe the stellar population out to the cluster's half-mass radius (r(h) similar or equal to 2.'2). The colour - magnitude diagram (CMD) that we derive in this way reveals a narrow and well defined MS extending down to the 5 sigma detection limit at V similar or equal to 25, or about 6 magnitudes below the turn-off, corresponding to stars of similar to 0.25 M-.. The luminosity function (LF) obtained with these data shows a marked radial gradient, in that the ratio of lower- and higher-mass stars increases monotonically with radius. The mass function (MF) measured at the half-mass radius, and as such representative of the cluster's global properties, is surprisingly flat. Over the range 0.4-0.8 M-., the number of stars per unit mass follows a power-law distribution of the type dN/dm proportional to m(0), where, for comparison, Salpeter's IMF would be dN/dm proportional to m(-2.35). We expect that such a flat MF does not represent the cluster's IMF but is the result of severe tidal stripping of the stars from the cluster due to its interaction with the Galaxy's gravitational field. Our results cannot be reconciled with the predictions of recent theoretical models that imply a relatively insignificant loss of stars from NGC6218 as measured by its expected very long time to disruption. They are more consistent with the orbital parameters based on the Hipparcos reference system that imply a much higher degree of interaction of this cluster with the Galaxy than assumed by those models. Our results indicate that, if the orbit of a cluster is known, the slope of its MF could be useful in discriminating between the various models of the Galactic potential.