Galactic Archaeology with Cool Stars and Brown Dwarfs in Globular Clusters

By analogy to traditional archaeology, Galactic Archaeology aims to uncover the history of our galaxy through the fossilized record, retained in the chemical makeup of its stars. The chemical abundances are typically inferred from the line strengths in medium- and high-resolution spectra. Exploring the early phases of galactic evolution is especially challenging due to the scarcity of sufficiently old stars in the solar neighborhood, which restricts spectroscopic studies to bright red giants. Nevertheless, the lowest-mass main sequence stars and brown dwarfs are essential in measuring the stellar mass function and the mass dependency of chemical evolution over the age of the Milky Way. The dominant molecular opacity in low-temperature stellar and substellar atmospheres makes their photometric colors particularly sensitive to element abundances, enabling the inference of chemical compositions in large coeval populations from color-magnitude diagrams alone. Milky Way globular clusters are ideal targets for this analysis as they host the oldest and largest coeval stellar populations in the galaxy.

I present a method for determining the chemical composition and mass function of globular clusters from space-based main sequence and brown dwarf photometry. My analysis is based on new grids of evolutionary models and model atmospheres that are tailored to individual globular clusters and account for key features of cool stars including molecular chemistry, atmosphere-interior coupling and formation of condensates. I apply my method to the HST observations of globular clusters ω Centauri, NGC 6752 and 47 Tucanae, as well as new JWST observations of the latter. The color-magnitude diagram of NGC 6752 presents a rare case of main sequence trifurcation at the lowest stellar masses. I analyze all three sub-populations individually for differences in their mass functions and chemical compositions. In 47 Tucanae, the inferred spread in light element abundances at the faint end of the main sequence is compared to the spectroscopic measurements of red giants. The unprecedented photometric sensitivity of JWST allows for the first constraints on the age of the cluster from the cooling curves of its brown dwarf members.