What is a globular cluster? An observational perspective

TL;DR

This review synthesizes observational data on globular clusters, emphasizing their chemical evolution, multiple populations, and the influence of initial mass, metallicity, and environment, highlighting challenges for formation models.

Contribution

It provides a comprehensive observational perspective on globular clusters, integrating data on chemical evolution, multiple populations, and environmental effects to inform formation theories.

Findings

Chemical evolution depends mainly on initial mass and metallicity.

Multiple populations affect various regions of the colour-magnitude diagram.

Constraints include lithium abundances, cluster dynamics, and binary star frequencies.

Abstract

Globular clusters are large and dense agglomerate of stars. At variance with smaller clusters of stars, they exhibit signs of some chemical evolution. At least for this reason, they are intermediate between open clusters and massive objects such as nuclear clusters or compact galaxies. While some facts are well established, the increasing amount of observational data are revealing a complexity that has so far defied the attempts to interpret the whole data set in a simple scenario. We review this topic focusing on the main observational features of clusters in the Milky Way and its satellites. We find that most of the observational facts related to the chemical evolution in globular clusters are described as being primarily a function of the initial mass of the clusters, tuned by further dependence on the metallicity - that mainly affects specific aspects of the nucleosynthesis processes involved - and on the environment, that likely determines the possibility of indepenedent chemical evolution of the fragments or satellites, where the clusters form. We review the impact of multiple populations on different regions of the colour-magnitude diagram and underline the constraints related to the observed abundances of lithium, to the cluster dynamics, and to the frequency of binaries in stars of different chemical composition. We then re-consider the issues related to the mass budget and the relation between globular cluster and field stars. Any successful model of globular cluster formation should explain these facts.