VLT FORS-1 observations of NGC 6397: Evidence for mass segregation

We present (V, V - I) VLT-FORS1 observations of the Galactic Globular Cluster NGC 6397. We derive accurate color-magnitude diagrams and luminosity functions (LFs) of the cluster Main Sequence (MS) for two fields extending from a region near the centre of the cluster out to similar or equal to10'. The photometry of these fields produces a narrow MS extending down to V 27, much deeper than any previous ground based study on this system and comparable to previous HST photometry. The V, V - I CMD also shows a deep white dwarf cooling sequence locus, contaminated by many field stars and spurious objects. We concentrate the present work on the analysis of the MSLFs derived for two annuli at different radial distance from the center of the cluster. Evidence of a clear-cut correlation between the slope of the observed LFs before reaching the turn-over, and the radial position of the observed fields inside the cluster area is found. We find that the Us become flatter with decreasing radius (x similar or equal to 0.15 for 1' < R-1 < 5.5' x similar or equal to 0.24 for 5.5' < R-2 < 9.8' core radius, r(c) = 0.05'), a trend that is consistent with the interpretation of NGC 6397 as a dynamically relaxed system. This trend is also evident in the mass function.