The environment of formation as a second parameter for globular cluster classification

TL;DR

This study uses multivariate analysis to classify 54 Galactic globular clusters, revealing that their environment influences their formation and properties, with inner halo clusters being more massive and self-enriched.

Contribution

It introduces a new classification based on environment, linking globular cluster properties to their formation regions within the Galaxy.

Findings

Three distinct cluster groups identified through cladistic analysis.

Inner halo clusters are more massive and helium-enriched.

Outer halo clusters are less massive and formed in different environments.

Abstract

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.