The environment of formation as a second parameter for globular cluster
  classification
  We perform an evolutionary multivariate analysis of a sample of 54 Galactic
globular clusters with high-quality colour-magnitude diagrams and
well-determined ages. The four parameters adopted for the analysis are:
metallicity, age, maximum temperature on the horizontal branch, and absolute V
magnitude. Our cladistic analysis breaks the sample into three novel groups. An
a posteriori kinematical analysis puts groups 1 and 2 in the halo, and group 3
in the thick disc. The halo and disc clusters separately follow a
luminosity-metallicity relation of much weaker slope than galaxies. This
property is used to propose a new criterion for distinguishing halo and disc
clusters. A comparison of the distinct properties of the two halo groups with
those of Galactic halo field stars indicates that the clusters of group 1
originated in the inner halo, while those of group 2 formed in the outer halo
of the Galaxy. The inner halo clusters were presumably initially the most
massive one, which allowed the formation of more strongly helium-enriched
second generation stars, thus explaining the presence of Cepheids and of very
hot horizontal-branch stars exclusively in this group. We thus conclude that
the "second parameter" is linked to the environment in which globular clusters
form, the inner halo favouring the formation of the most massive clusters which
subsequently become more strongly self-enriched than their counterparts of the
galactic outer halo and disc.
