Signatures of the r-Process in Ancient Stellar Populations Using Barium Abundances

Abstract

For over sixty years scientists have known that a large percentage of heavy elements are created by the rapid neutron-capture process (r-process). However, a clear picture of where the r-process occurs has remained elusive. Many astrophysical origins have been proposed -- each with a range of possible chemical yields and rates. Discovering which origin (or combinations of origins) truly produce the heavy elements we see on Earth is a daunting task. This thesis seeks to provide observational constraints to pinpoint the dominant origin of the r-process. The majority of this thesis uses Galactic Archaeology to look for r-process signatures in ancient stellar populations (e.g., dwarf galaxies and globular clusters). These ancient stellar populations provide the clearest experiments to observe how quickly and how much r-process was created. The r-process signature we observe is the amount of barium in individual red giant branch stars in these stellar populations. Chapter 2 discusses how these barium measurements are made from individual extragalactic stars and presents the largest catalog of barium abundances in dwarf galaxies to date. Chapter 3 compares the r-process signature -- barium -- to other elements (e.g., magnesium and iron) in the same galaxy to see how the timescale of r-process enrichment compares to the other abundances (whose origins are known). This analysis found that the r-process timescale was more delayed than core-collapse supernovae. This points to neutron star mergers (NSMs) as the dominant source of r-process in the early history of dwarf galaxies. Chapter 4 uses a galactic chemical evolution model to test what r-process timescales, yields, and rates are needed to recreate the observations presented in Chapter 2. Preliminary results indicate that NSMs must be included in order for the model to match the observations. In addition, Chapter 4 presents what yield of barium is needed from NSMs to recreate the observations. Chapter 5 tests if the stars in the globular cluster M15 were enriched by the r-process after they were born. M15 has an unusual abundance pattern with ~ 1 dex variation in r-process abundances even though most other elements, including iron, do not show a variation. New measurements of barium abundances in main sequence and red giant branch stars of M15 show that the stars were born with their r-process enrichment. This means that an r-process event occurred quickly after the cluster was born -- while it was still forming stars -- and resulted in uneven enrichment. Finally, Chapter 6 presents a solution to one of the technical challenges in locating the sites of r-process nucleosynthesis. Chapter 6 describes how to accurately measure the position and orientation of the CCDs in Zwicky Transient Facility's (ZTF's) camera. ZTF is a transient survey that -- among other science goals -- searches for the electromagnetic counterpart of NSM detections with LIGO. The work included in this chapter increased the survey efficiency of ZTF, which will aid ZTF in localizing transient events, including NSMs. Following up NSMs found by LIGO can provide direct measurements of the amount of r-process material created by NSMs. Altogether, this thesis has made strides to identifying the origin of the r-process. Chapters 3 and 4 identify NSMs as the dominant source of r-process elements in dwarf galaxies. However, Chapter 5 found that globular cluster M15 needs a r-process event to occur quickly -- quicker than is typically expected from a NSM. The observational constraints that have resulted from this thesis provide important clues to where the heaviest elements are made.