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 \aas@@fstack { \prime } 4 \times 3 \aas@@fstack { \prime } 4 around the cluster centre and probe the stellar population out to the cluster ’ s half-mass radius ( r _ { h } \simeq 2 \aas@@fstack { \prime } 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 \simeq 25 , or about 6 magnitudes below the turn-off , corresponding to stars of \sim 0.25 { M } _ { \odot } .
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 } _ { \odot } , the number of stars per unit mass follows a power-law distribution of the type dN / dm \propto m ^ { 0 } , where , for comparison , Salpeter ’ s IMF would be dN / dm \propto 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 can not be reconciled with the predictions of recent theoretical models that imply a relatively insignificant loss of stars from NGC 6218 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 .