Multiple stellar populations in young and intermediate-age, massive clusters in the Magellanic Clouds
There are many mysteries in the Universe, but one unanswered question is, how did the globular star clusters form? What is the driving mechanism for them to become dense agglomerations of hundreds of thousands of stars? Globular clusters host some of the oldest stars in the Universe. To understand galaxy formation and evolution, these globular clusters are ideal laboratories.
The traditional picture of globular clusters is that they are coeval and all of their stars have the same composition. This concept is commonly known as a ‘simple’ stellar population. However, in the last decade new evidence of chemical inhomogeneities has been detected in nearly all globular clusters. Nearly all old and massive globular clusters in the Milky Way and its satellite galaxies were found to host several stellar populations. This phenomenon is typically known as multiple stellar populations (MPs). High-precision and deep photometry have been useful in detecting chemical patterns in globular clusters. Understanding the mechanism behind the formation of MPs and its impact on the evolution of globular clusters is crucial to investigate formation theories of stars and of the clusters themselves. Additionally, the formation and evolution of galaxies can be understood in detail if we know how MPs are formed in globular clusters.
The Milky Way is comparatively old with a low level of active star formation, and it hosts only a handful of massive, intermediate-age star clusters. These clusters are hard to observe because they are hidden behind many magnitudes of extinction. Intermediate-age star clusters in the Magellanic Clouds, on the other hand, offer a more easily accessible opportunity to understand star formation and its impact on the chemical evolution of the host galaxy. To date, detailed chemical abundances of stars in intermediate-age Magellanic Cloud clusters based on high-resolution spectra are limited to only a few stars in just a few star clusters. Despite such promising results, these small numbers are as yet insufficient for robust analyses of the chemical enrichment and MP characteristics of intermediate-age clusters as a population.
Multiple scenarios have been proposed to explain the origin of chemical inhomogeneities in star clusters. Most theoretical models propose that they are the result of multiple events of star formation. In such models, a first generation of stars forms from the collapse of a giant molecular cloud. This molecular cloud is homogeneous in its chemical composition. The material expelled from the massive stars through their winds from this first generation sinks to the centre of the cluster. This provides material to form a second generation of stars. However, this second generation is formed with a different chemical composition, displaying enhancements in helium and nitrogen abundances. This theory looks convincing, painting a complete picture and reasoning behind the formation of MPs. However, it fails to reproduce many observed properties of MPs at various evolutionary stages. Therefore, the formation mechanism of the origin of MPs remains an open and unanswered question.
The main goal of this thesis is to expand the search for MPs to star clusters that are significantly younger than the ancient globular clusters. The sample of star clusters I have selected in this thesis spans ages between ~ 1.7 and 3.0 Gyr. I report the first detection of MPs in the youngest star cluster known to host MPs to date, aged 1.7 ± 0.02 Gyr, showing that the MP phenomenon is not restricted only to older clusters. My thesis suggests that age of a cluster plays an important role in the establishment of MPs. Results from my thesis will have a major impact on our understanding of MP and cluster formation scenarios, and of the importance of the galactic environment.
