Innovative Sequencing of Multiple Stellar Populations in Star Clusters

• Applicant: Privatdozentin Dr. Geneviève A. Parmentier
• Subject Area: Astrophysics and Astronomy
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 515414180

Most stars in the Universe form in star clusters. Our research community used to think of them as groups of stars of the same age and chemical composition. Over the past twenty years, however, numerous observations have revealed a very different picture. It is now clear that young star clusters sometimes experience successive episodes of star formation. Additionally, inside the oldest clusters of the Universe, the chemical composition of the stars varies, with some stars being poorer in oxygen but richer in nitrogen and sodium. This matches the chemical composition of the winds released by stars 10,000 times more massive than the Sun, the so-called Super Massive Stars. To form one of them, many stars must collide and merge together, which can happen in the dense central regions of compact and massive clusters. There a Super Massive Star can form fast enough to alter the chemical composition of the gas inside the cluster, gas out of which stars are still forming. The cluster then contains two main groups of stars, depending on whether or not their chemical composition has been affected by the Super Massive Star. In many old star clusters, however, each group consists of several subgroups whose formation history remains puzzling. A key objective of my project is to identify along which time-sequence and in which regions of their parent cluster these various stellar subgroups form. To do so, I will model the star formation history and chemical evolution of clusters hosting a Super Massive Star. This is crucial to understand the distributions of stars as a function of their chemical composition. Such distributions vary widely among old clusters, being sometimes smooth, sometimes clumpy. Do we have an imprint of cluster star formation histories there? How much does the chemical composition of stars depend on the mass, size, density profile and star formation efficiency of the gas cloud in which the cluster forms? The few young, compact and massive clusters of our Galaxy often contain a greater proportion of massive stars than smaller clusters do. Is it the case for the model clusters obtained above? In this project I will also establish their stellar mass spectrum and how it varies over the course of cluster formation, for the proportion of massive stars in clusters has important consequences for both their short-term and long-term evolution, and for the evolution of their host galaxy.

• DFG Programme: Research Grants