In search of massive single-population Globular Clusters
  Most Globular Clusters so far examined host (at least) two stellar
populations. This feature requires a two--step process, in which the nuclearly
processed matter from a first generation (FG) of stars gives birth to a second
generation (SG) bearing the fingerprint of a fully CNO-cycled matter. Since the
present population of most globular clusters is made up largely of SG stars, a
substantial fraction of the FG (>~90%) must be lost. Nevertheless, two types of
clusters dominated by a simple stellar population (FG clusters) should exist:
either clusters initially too small to be able to retain a cooling flow and
form a SG (FG-only clusters), or massive clusters that could retain the CNO
processed ejecta and form a SG, but were unable to lose a significant fraction
of their FG (mainly-FG clusters). We attempt a classification of FG clusters,
based on the morphology of their horizontal branches (HBs), as displayed in
photomectric catalogues for 106 clusters. FG candidates are the clusters in
which the HB can be reproduced by the evolution of an almost unique mass. <20%
of clusters with [Fe/H]<-0.8 appear to be FG, but only ~10% probably had a mass
sufficient to form at all an SG. This small percentage confirms on a wider
database that the SG is a dominant constituent of today's clusters, suggesting
that its formation is an ingredient necessary for the survival of globular
clusters during their dynamical evolution in the Galactic tidal field. Pal3
turns out to be a good example of FG-only cluster. HB simulations and space
distribution of its components, indicate that M 53 is a "mainly-FG" cluster.
Mainly-FG candidates may be also NGC5634, NGC5694 and NGC6101. In contrast, NGC
2419 contains >30% of SG stars, and its present dynamical status bears less
information on its formation process than the analysis of the chemical
abundances of its stars and of its HB morphology.
