The globular cluster NGC 6642: Evidence for a depleted mass function in
  a very old cluster
  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 ~ 6 mags below
the turn-off in m_F606W. A theoretical isochrone fitting was performed and
evolutionary parameters were obtained, such as the metallicity [Fe/H] = -1.80
+/- 0.2 and age log(Age) = 10.14 +/- 0.05. We confirm that NGC 6642 is located
in the Galactic bulge, with a distance to the Sun d_{\odot} = 8.05 +/- 0.66 ~
kpc$ and the reddening E(B-V) = 0.46 +/- 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 ~ 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.
