Structural parameters of 11 faint Galactic globular clusters derived with 2MASS

Universidade Federal do Rio Grande do Sul, Departamento de Astronomia CP 15051, RS, Porto Alegre 91501-970, Brazil e-mail: [charles;bica]@if.ufrgs.br

Received: 17 September 2007
Accepted: 7 November 2007

Abstract

Context.Structural parameters and the total M_V magnitude are important properties for the characterisation of individual globular clusters. With the growth in statistics, especially of the intrinsically faint objects, the collective properties of the Galactic globular cluster system will be better defined, leading to a deeper understanding of the Galaxy formation processes.

Aims.We determine the structural parameters of 11 faint Galactic globular clusters that, in most cases, had not been previously studied in this context: IC 1257, Lyngå 7, Terzan 4, Terzan 10, BH 176, ESO 452-SC11, ESO 280-SC08, 2MASS-GC01, 2MASS-GC02, GLIMPSE-C01, and AL 3. They are projected not far from the central region of the Galaxy. Field-star contamination is significant in the colour-magnitude diagrams. Half of the sample has an absorption 7, in some cases reaching 15.

Methods.Stellar radial number-density and surface-brightness profiles were built with 2MASS photometry that, for the present clusters, corresponds to giant-branch stars. Field-star decontamination is essential for clusters in dense fields, so an algorithm that we had previously developed for open clusters in rich fields was employed to better define cluster sequences. With decontaminated photometry we also computed the total M_V of four such globular clusters, using M 4 as a template. King-like functions were fitted to the radial profiles, from which the core, half-light, half-star count, and tidal radii were derived, together with the concentration parameter. The derived parameters were then compared to equivalent ones available in the literature for other Galactic globular clusters.

Results.Compared to massive globular clusters, those in the present sample have smaller tidal and larger core radii for a given Galactocentric distance, which is consistent with rather loose structures. Globular cluster radii derived from number-density and surface-brightness profiles have similar values. The present magnitude estimates are M_V ≈ -4.9 (ESO 280-SC08), M_V ≈ -5.8 (2MASS-GC01), and M_V ≈ -5.6 (2MASS-GC02). We also estimate M_V ≈ -6.1 for GLIMPSE-C01, which is somewhat less luminous than previously given. The density profiles of Tz 4 and 2MASS-GC01 show evidence of post-core collapse clusters.

Conclusions.The structural parameters and luminosity of most of the faint globular clusters dealt with in this paper are consistent with those of Palomar-like (low mass and loose structure) globular clusters. This work helps to improve the coverage of the globular cluster parameter space.