Not-so simple stellar populations in nearby, resolved intermediate-age massive clusters
It is assumed that during stellar cluster formation, the stellar members form at the same time and inherit the chemistry of the progenitor cloud. Hence, this simplistic model of cluster formation assumes all cluster members to be a ‘simple stellar population’, which can be defined by a single-age isochrone. However, the simple stellar population model was challenged when it was observed that old globular clusters host chemically distinct populations of stars.
The presence of chemically distinct subpopulations in globular clusters came to be known as ‘multiple populations’, often interpreted from colour-magnitude diagram features like split/spread main sequences, red giant branches, sub-giant branches and horizontal branches. Several aspects regarding the formation, triggering and evolution of multiple populations within a globular cluster remain a mystery. However, cluster age and mass have been identified to play a role in the presence of multiple populations within them. Clusters older than 2 Gyr and more massive than 104𝑀⊙ have been found to host multiple populations.
We aimed to study intermediate-age massive clusters around the critical age limit of 2 Gyr to constrain the presence of multiple populations as a function of cluster age. Do they exhibit deviations from the simple stellar population model? If so, are there chemical abundance spreads within these clusters? Or is the deviation from the simple stellar population model due to a difference in rotation rates or some other factor? We focused on the red giant branch stars of our intermediate-age cluster sample. Our study found the presence of multiple populations in the intermediate-age massive clusters, NGC 2121 and in the LMC field surrounding the cluster NGC 1953. Is this due to an actual abundance spread? The answer to this question lies in a proposed spectroscopic study of our targets.
