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

Published online by Cambridge University Press:  18 January 2010

Eduardo Balbinot ,
Basílio X. Santiago ,
Eduardo Bica  and
Charles Bonatto
Eduardo Balbinot
Affiliation:
Departamento de Astronomia, Universidade Federal do Rio Grande do Sul, Porto Alegre, Brazil email: [email protected]
Basílio X. Santiago
Affiliation:
Departamento de Astronomia, Universidade Federal do Rio Grande do Sul, Porto Alegre, Brazil email: [email protected]
Eduardo Bica
Affiliation:
Departamento de Astronomia, Universidade Federal do Rio Grande do Sul, Porto Alegre, Brazil email: [email protected]
Charles Bonatto
Affiliation:
Departamento de Astronomia, Universidade Federal do Rio Grande do Sul, Porto Alegre, Brazil email: [email protected]
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Abstract

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We present photometry for the globular cluster NGC 6642 using the F606W and F814W filters with the ACS/WFC third-generation camera aboard the Hubble Space Telescope. The colour–magnitude diagram shows sources reaching ≈ 6 mag below the turnoff in mF606W. Theoretical isochrone fitting was performed and evolutionary parameters were obtained, including the metallicity [Fe/H] = −1.80 ± 0.2 dex and age, log(age/yr) = 10.14 ± 0.05. We confirm that NGC 6642 is located in the Galactic bulge, at a distance of d⊙ = 8.05±0.66 kpc and suffers from a reddening of E(B − V) = 0.46 ± 0.02 mag. These values are in general agreement with those of previous authors. 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 stellar numbers as a function of increasing mass. This has been observed previously in other globular clusters and is also the result of N-body simulations of stellar systems which have reached ≃90% of their lifetime and are subjected to strong tidal effects. We thus conclude that NGC 6642 is a very old, highly evolved globular cluster. Its current location close to perigalacticon, at only 1.4 kpc from the Galactic Centre, may contribute to this high level of dynamical evolution and stellar depletion.

Keywords

globular clusters: generalglobular clusters: individual (NGC 6642)Galaxy: structure
Type
Contributed Papers
Information
Proceedings of the International Astronomical Union , Volume 5 , Symposium S266: Star clusters: basic galactic building blocks throughout time and space , August 2009 , pp. 357 - 360
Copyright
Copyright © International Astronomical Union 2010

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