We present photometry for the globular cluster NGC 6642 using the F606W and F814W filters with the ACS/WFC third generation camera on board of Hubble Space Telescope .
The Colour Magnitude Diagram shows sources reaching \approx 6 ~ { } mags below the turn-off in m _ { F 606 W } .
A theoretical isochrone fitting was performed and evolutionary parameters were obtained , such as the metallicity \mbox { $ [ Fe / H ] $ } = -1.80 \pm 0.2 and age \mbox { $log ( \tau ( yrs ) ) $ } = 10.14 \pm 0.05 .
We confirm that NGC 6642 is located in the Galactic bulge , with a distance to the Sun \mbox { $d _ { \odot } $ } = 8.05 \pm 0.66 ~ { } kpc and the reddening \mbox { $E ( B - V ) $ } = 0.46 \pm 0.02 .
These values are in general agreement with those of previous authors .
About 30 blue stragglers were found within the central 1.6 pc of NGC 6642 .
They are strongly concentrated to the very central regions .
The cluster displays a well-developed horizontal branch , with a much redder morphology than that of typical old halo globular clusters of similar metallicity .
Completeness corrected luminosity and mass functions were obtained for different annuli centred on NGC 6642 .
Their spatial variation indicates the existence of mass segregation and depletion of low mass stars .
Most striking is the inverted shape of the mass function itself , with an increase in number as a function of increasing mass .
This has been previously observed in other globular clusters and is also the result of N-body simulations of stellar systems which have undergone \simeq 90 \% of their lifetime and which are subjected to strong tidal effects .
We also analysed the density profile and concluded that NGC 6642 has a collapsed core , provided completeness effects are correctly accounted for .
We thus conclude from independent means that NGC 6642 is a very old , highly-evolved , core-collapsed globular cluster with an atypical HB morphology .
Its current location close to perigalactic , at only 1.4 kpc from the Galactic centre , may contribute to this high level of dynamical evolution and stellar depletion .