UC San Diego

The chemical diversity of our galaxy is owed to multiple generations of stars that converted the primordial mixture of hydrogen and helium into ~100 elements that make up the periodic table. The first stars to form after the Big Bang (Population III) were nearly metal-free and are believed to have been far more massive, luminous and short-lived than their later descendants. It is therefore expected that none of the Population III stars in the Milky Way survived to present day and their properties are highly debated. By contrast, the oldest metal-poor (Population II) stars that formed during or shortly after the era of Population III dominance may still be found in the galactic halo. Large numbers of those ancient stars are contained within the Milky Way globular clusters that are of particular interest to studies of stellar populations due to their coeval nature. Recent observations have confirmed that globular clusters undergo complex evolution and host multiple chemically distinct stellar populations. The physical origin of this unique feature remains largely unexplained, in part, because detailed composition measurements are only available for the most massive, spectroscopically accessible members.

In order to study the early chemical evolution of the universe, I developed a method to extract the fundamental properties of chemically peculiar stellar populations from multiband photometry. My approach uniquely incorporates the calculation of new evolutionary stellar models and model atmospheres for every considered chemical composition, thereby capturing the exact relationships between individual atomic abundances and photometric colors. The computational efficiency of the fitting process is attained by identifying the components of the stellar models that are most sensitive to particular elements, and recalculating them only when the abundances of those elements are updated.

In this dissertation, I describe applications of my modelling framework to the oldest stars in the universe from the elusive Population III to the ancient members of the nearby globular clusters ω Centauri and 47 Tucanae. I present predicted colors of metal-free stars at high redshift as they may be observed by the recently launched James Webb Space Telescope (JWST) under favorable gravitational lensing. These future observations or lack of thereof will provide strict constraints on the mass function, formation and supernova yields of the first stars. My models reproduce the entire color distribution of the main sequence stars in the globular cluster 47 Tucanae and provide a theoretical baseline for the ongoing observing campaigns with JWST that are expected to uncover the substellar cooling sequence of the cluster. The new theoretical isochrones, tailored to the chemical composition of 47 Tucanae, allow the first detailed analysis of the variation in chemical abundances with stellar mass, which is essential to determine the origin of multiple populations in globular clusters.