Multiple Stellar Populations in Globular Clusters

TL;DR

Globular Clusters show star-to-star element variations indicating multiple stellar populations, but current models fail to fully explain these observations, highlighting the need for new theories.

Contribution

This review summarizes current observations of multiple populations in GCs and critically assesses existing models, emphasizing their shortcomings and the necessity for alternative explanations.

Findings

Most GCs exhibit multiple stellar populations with elemental variations.

Existing models, especially those involving multiple star formation epochs, fail to explain all observations.

The puzzle of multiple populations in GCs remains unsolved, requiring new theoretical approaches.

Abstract

Globular Clusters (GCs) exhibit star-to-star variations in specific elements (e.g., He, C, N, O, Na, Al) that bare the hallmark of high temperature H burning. These abundance variations can be observed spectroscopically and also photometrically, with the appropriate choice of filters, due to the changing of spectral features within the band pass. This phenomenon is observed in nearly all of the ancient GCs, although, to date, has not been found in any massive cluster younger than 2~Gyr. Many scenarios have been suggested to explain this phenomenon, with most invoking multiple epochs of star-formation within the cluster, however all have failed to reproduce various key observations, in particular when a global view of the GC population is taken. We review the state of current observations, and outline the successes and failures of each of the main proposed models. The traditional idea of using the stellar ejecta from a 1st generation of stars to form a 2nd generation of stars, while conceptually straight forward, has failed to reproduce an increasing number of observational constraints. We conclude that the puzzle of multiple populations remains unsolved, hence alternative theories are needed.