2009MNRAS.398.1706F

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.