2026 Theses Doctoral

From the First Stars to the Milky Way: Fingerprints of Chemical Enrichment Across Cosmic Time

Mead, Jennifer

The chemical elements that make up the stars, planets, and even life itself, were forged by successive generations of stars. Of the 118 known elements, 94 of them originate from cosmic processes, of which only hydrogen, helium, and trace amounts of lithium were formed in the Big Bang; all others are products of stellar evolution and death. As stars end their lives, they deposit ``metals,'' or any element heavier than helium, into their surroundings, enriching the gas from which new stars and planets form, and driving the chemical evolution of the Universe. The relative abundance of elements not only shape the conditions for life on Earth, but encode a record, traceable over cosmic time, that describes the history of our Solar System, the Milky Way galaxy, and our Universe. Evolution is defined as the gradual development of something, particularly from a simple to a complex form. As such, the study of the chemical evolution of our Universe requires an understanding of the beginning and the end, from the first stars to the present day.

This dissertation presents a series of works aimed at studying the processes that drive chemical evolution starting with the first stars a few hundreds of millions of years after the Big Bang, to the signatures of chemical evolution that are observed at the present-day in stars of the Milky Way and Local Group.

In Chapters 2 and 3, we use star-by-star cosmological simulations to study the impact of the first enrichment events on early galaxies and subsequent star formation. In particular, in Chapter 2, we study the loss of metals from early galaxies and identify a characteristic halo mass upon which galaxies are capable of retaining metals and hosting ongoing enriched star formation. Chapter 3 uses the Mahalanobis distance to explore the chemical homogeneity of gas in star-forming regions relative to the stars that form from it. We find that during the earliest epochs of star formation, the chemical abundances of gas only remain similar to new stars within 100 parsecs of a site of star formation and within 7 million years of a star formation event. This has implications for our ability to chemically tag the oldest, most metal-poor stars the still live in our galaxy.

In Chapters 4 and 5, we turn to data-driven and statistical methods to interpret signatures of chemical evolution in stellar abundance observations and to place constraints on simulations which can be used to understand the physical drivers of chemical evolution. In Chapter 4, we measure the intrinsic scatter of element abundances among mono-metallicity stars in Local Group dwarf galaxies, Milky Way globular clusters, and the Milky Way stellar halo. Intrinsic scatter among element abundances can arise from various processes including the inhomogeneous mixing of gas, variations of the ratio of different enrichment channels, mass-, metallicity-, and time-dependent yields, and environmental variations in and stochastic sampling of the initial mass function. Measurements of the intrinsic scatter in different systems will help to constrain which of these processes is dominant in simulations.

Finally, in Chapter 5, we develop a framework for reducing the dimensionality of chemical space, identifying four key elements that trace major nucleosynthetic processes and are able to describe 30 element abundances of solar twins in the Solar neighborhood to within 5%. This work motivates the need for high-precision, high-resolution stellar abundance surveys in order to realize chemical tagging. Together, the works in this dissertation highlight the connections that can be made between methodologies and across cosmic time in the pursuit of understanding the evolution of our Universe through its chemistry.