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Astron. Astrophys. 327, 1004-1016 (1997)

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1. Introduction

The recent advances in our understanding of the structure and evolution of Galactic globular clusters (GCs) have been possible thanks to the advent of accurate CCD photometry. However, till few years ago, CCD photometry was limited to the internal parts of GCs due to the small fields of the detectors. All the information relative to the outer regions and to the tidal radius [FORMULA], arise from visual (by eye) stellar counts made on Schmidt plates, especially by King and collaborators (King et al. 1968; Trager et al. 1995) This methodology of investigation suffers from various problems and statistical biases; we list some of them:

  • The limiting magnitude of photographic plates, which is generally too bright to permit the investigation of the radial distribution of stars in an appropriate mass range;
  • the high uncertainty in the evaluation of background stellar contamination;
  • an insufficient crowding/completeness correction.

All the more recent models of dynamical evolution need to make assumptions on the mass function, on the effects due to the radial anisotropy of the velocity distribution, and on the mass segregation which, in principle, could be determined observationally.

Fostered by this lack of observational data, five years ago we started a long term project using one of the largest field CCD cameras available, EMMI at the NTT, to obtain accurate stellar photometry in two bands, V and I, over at least a full quadrant in a number of GCs. The sample was selected taking into account the different morphological types and the different positions in the Galaxy. The principal aim was to map the stellar distribution from the central part out to the outer envelope (beyond the formal tidal radius, for a better estimate of the field stars contamination and in order to investigate on the possible presence of tidal tails), with a good statistical sampling of the stars in distinct zones of the color magnitude diagram (CMD) and of different masses.

The use of CCD star counts, instead of photographic Schmidt plates for which the only advantage is still to give a larger area coverage (Grillmair et al. 1995; Lehman & Sholz 1997, for works on this subject), allows us to go deeper inside the core of the clusters, to better handle photometric errors and completeness corrections and to reach a considerably fainter magnitude level. CCDs also allows in the case of high concentration clusters to complement star counts of the central part with aperture photometry of short exposures images (Saviane et al. 1997).

An important byproduct of this study is the photometry of a significant number of stars in all the principal sectors of the CMD. This sample is of fundamental importance to test modern evolutionary stellar models (Renzini & Fusi Pecci 1988). In all cases the CMD extends well below the turn off of the main sequence. This permits us to estimate the effect of mass segregation for masses from the TO mass ([FORMULA]) down to [FORMULA].

So far, we have collected data for a total of 19 clusters. Same of them have already been reduced and analyzed. In this work we present the analysis of the star counts of the globular cluster NGC 6809=M 55. Other clusters, for which we have already given a first report elsewhere (Zaggia et al. 1995; Veronesi et al. 1996; Saviane et al. 1995; Rosenberg et al. 1996), will be presented in future works (Saviane et al. 1997; Rosenberg et al. 1997).

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© European Southern Observatory (ESO) 1997

Online publication: April 6, 1998