Feedback on multiple stellar populations in globular clusters

Supervisors: Alice Zocchi, Glenn van de Ven and Elke Pilat-Lohinger (Department of Astrophysics), in collaboration with J. Alves and S. Meingast (Department of Astrophysics)

Funding Situation: potentially via a VISESS PhD fellowship

Project outline: Globular clusters (GCs) have long been described as round, isotropic and non-rotating stellar systems with stars of the same age and chemical composition. However, this simple picture is incorrect and in particular the discovery of multiple populations (MPs) has raised unanswered questions about the formation of GCs, their chemo-dynamical evolution, and their contribution to galaxies through tidal stripping.

MPs in GCs have similar metallicities, but some stars are enriched in Na (N) and depleted in O (C). While one suggested interpretation is that second-generation (enriched) stars formed from material polluted by the ejecta of first-generation (pristine) stars, none of the current scenarios can reproduce all observational constraints. Specifically, all the proposed polluters fail with respect to the so-called mass budget problem: most of the stars in GCs today belong to the enriched component, so the number of pristine stars originally had to be an order of magnitude larger to allow for sufficient pollution, but there is no plausible explanation of how most of them disappeared.

By using theoretical models, simulations, and observations, we will explore whether the mass budget problem can be solved through a more efficient pollution from dynamical mixing of stars with gas in GCs in the early Universe. We will set up realistic configurations for primordial GCs, and then investigate the interplay between stellar dynamics and feedback, by combining dynamical models and N-body simulations with prescriptions for the pollution mechanisms. We will compare the results obtained from these theoretical investigations to data of star clusters hosting MPs.