THE STELLAR CONTENT AND STRUCTURE OF THE GLOBULAR-CLUSTER NGC-5053

Five overlapping fields in the metal poor globular cluster NGC 5053 have been observed with the CCD system at the Canada-France-Hawaii Telescope. The data have been used to obtain deep star counts, V less-than-or-equal-to 25.0, covering an east-west strip extending through the cluster center to close to the tidal radius. From the star counts alone, we determine a minimum mass-to-visual light ratio of M/L(V) = 0.68 +/- 0.07. This estimate is independent of kinematical observations and dynamical models of the cluster structure. The stellar luminosity functions at different radial positions within the cluster were converted to the corresponding mass functions using a 16 Gyr oxygen-enhanced isochrone of metallicity [Fe/H] = -2.03. Mass segregation is observed. For a mass function of the form n(m) is-proportional-to m-(1+x), we find that the core of the cluster has x congruent-to 0.8, whereas the outer region has x congruent-to 2.0. The global luminosity function for the stars with V less-than-or-equal-to 23.5 corresponds very well to a mass function with x = 1.5. The degree of mass segregation is broadly consistent with the predictions of multimass King models appropriate to NGC 5053. However, the observed mass segregation itself does not appear to provide a useful constraint on the amount of dark matter in the cluster. The projected mass and number density profiles for the cluster have been fitted with a series of representative multimass King models. Given that our surface brightness profile has a peculiarly high value (compared with an isotropic King model) in the most distant annulus defined in our study, we cannot conclusively rule out the possibility that the cluster may have an anisotropic velocity distribution that extends inside the cluster core. The recently determined value for the velocity dispersion of the cluster giants, together with our global mass function, leads to a global mass-to-visual-light ratio of M/L(V) almost-equal-to 1.2. This result, which is based on a multimass King model, implies that the cluster main-sequence mass function must be fairly abruptly truncated at the observational limit of our data, i.e., m congruent-to 0.35 m.. Finally we present a new color-magnitude diagram for NGC 5053 which reaches V = 23.5, well below the main-sequence turn-off. From a comparison with the main-sequence fiducial of M92, we estimate the distance modulus and reddening of NGC 5053 to be (m - M)V = 16.08 and E(B - V) = 0.06, respectively. This comparison also shows that NGC 5053 and M92 are the same age, independently of uncertainties in the distance moduli or reddenings of the clusters.