A critical assessment of models for the origin of multiple populations in globular clusters

TL;DR

This paper critically evaluates existing models for the origin of multiple populations in globular clusters and finds them all inconsistent with observations, suggesting the need for alternative explanations.

Contribution

It provides a comprehensive comparison of current models against observational data, demonstrating their failures and highlighting the necessity for new theories.

Findings

YMCs do not show multiple star-formation episodes

Chemical patterns in GCs are too variable for standard models

Mass budget problems cannot be solved by assuming higher initial cluster masses

Abstract

A number of scenarios have been put forward to explain the origin of the chemical anomalies (and resulting complex colour-magnitude diagrams) observed in globular clusters (GCs), namely the AGB, Fast Rotating Massive Star, Very Massive Star, and Early Disc Accretion scenarios. We compare the predictions of these scenarios with a range of observations (including young massive clusters (YMCs), chemical patterns, and GC population properties) and find that all models are inconsistent with observations. In particular, YMCs do not show evidence for multiple epochs of star-formation and appear to be gas free by an age of ~3 Myr. Also, the chemical patterns displayed in GCs vary from one to the next in such a way that cannot be reproduced by standard nucleosynthetic yields. Finally, we show that the "mass budget problem" for the scenarios cannot be solved by invoking heavy cluster mass loss (i.e. that clusters were 10-100 times more massive at birth) as this solution makes basic predictions about the GC population that are inconsistent with observations. We conclude that none of the proposed scenarios can explain the multiple population phenomenon, hence alternative theories are needed.