A wide-field view on multiple stellar populations in 28 Milky Way globular clusters

The majority of Galactic globular clusters (GCs) contain multiple stellar populations displaying specific chemical abundance variations. In particular, GCs generally contain a 'primordial' population with abundances similar to field stars, along with an 'enriched' population exhibiting light element anomalies. In this paper, we present a homogeneous and wide-view analysis of multiple stellar populations in 28 Galactic GCs. By using a combination of HST photometry together with wide-field, ground-based photometry we are able to analyse between 84 per cent and 99 per cent of all stars in each cluster. For each GC, we classify stars into separate sub-populations using the well-established C-UBI colour index, and investigate the spatial distributions of these populations. Our results show that dynamically young GCs can contain either centrally concentrated enriched or primordial populations, or no centrally concentrated population. Dynamically old GCs show fully mixed populations as expected. The existence of clusters born with centrally concentrated primordial (and homogeneously mixed) populations exacerbates the mass-budget problem facing many cluster formation scenarios. The diversity in these results also highlights the need for additional theories that can account for the wide variety of initial conditions that we find. We finally investigate the enriched star fraction as a function of different global parameters in our GC sample, using also data for young and low-mass clusters from the Small- and Large Magellanic Clouds and confirm earlier results that the enriched star fraction strongly correlates with the initial mass of a cluster.

Automated summary

This paper presents a comprehensive analysis of multiple stellar populations in 28 Galactic globular clusters. The results show that young clusters can have either enriched or primordial populations, or no concentrated population. Old clusters exhibit fully mixed populations as expected. The diversity in these results highlights the need for additional theories to explain the wide variety of initial conditions found. The paper also investigates the correlation between the enriched star fraction and the initial mass of a cluster.