The globular cluster NGC 6642: evidence for a depleted mass function in a very old cluster

We present photometry for the globular cluster NGC6642 using the F606W and F814W filters with the Advanced Camera for Surveys in the Wide Field Channel (ACS/WFC) third generation camera on board of Hubble Space Telescope. The colour-magnitude diagram shows sources reaching ~6mag 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[τ(yr)] = 10.14 +/- 0.05. We confirm that NGC6642 is located in the Galactic bulge, with a distance to the Sun d_solar = 8.05 +/- 0.66kpc 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 NGC6642. They are strongly concentrated to the very central regions. The cluster displays a well-developed horizontal branch (HB), 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 NGC6642. 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 per cent of their lifetime and which are subjected to strong tidal effects. We also analysed the density profile and concluded that NGC6642 has a collapsed core, provided completeness effects are correctly accounted for. We thus conclude from independent means that NGC6642 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.