School of Physics Thesis Defense

Presenter: Danielle Skinner
Title: Star Formation in the Early Universe: The First Stars and their Remnants
Date: Monday, April 17th
Time: 2:00 p.m.
Location: Boggs 1-44 CRA Viz-Lab
Via Zoom: https://gatech.zoom.us/j/95087208624?pwd=QU5GakhzV0lxWXBubjNxQ0JzcWtYZz09
Meeting ID: 950 8720 8624 / Passcode: 117132

Committee:
Dr. John Wise, School of Physics, Georgia Institute of Technology
Dr. Laura Cadonati, School of Physics, Georgia Institute of Technology
Dr. Gongjie Li, School of Physics, Georgia Institute of Technology
Dr. Surabhi Sachdev, School of Physics, Georgia Institute of Technology
Dr. Alexander Ji, Department of Astronomy and Astrophysics, University of Chicago

Abstract:
The exact evolution of elements in the universe, from primordial hydrogen and helium to heavier elements like gold and platinum, is still under scrutiny. The transformation from primordial to heavier elements starts with the first generation of stars, through nuclear fusion in their cores. These first stars, called Population III or Pop III, are the first radiating objects, formed from metal-free gas clouds in the very early universe. The supernova deaths of these stars leads to the enrichment of their local environments with new metals, and can leave behind neutron stars as remnants. These compact objects can end up in binary systems with other neutron stars, and eventually merge, which allows for the rapid neutron capture (r-process) to take place. This process is responsible for half of the elements heavier than iron, some of which end up enhancing the next generation of stars with this r-process material. These r-process enhanced stars, seen in the universe in ultra-faint dwarf galaxies like Reticulum II, can give us insight into the stellar ancestors of these stars. With the launch of JWST, we may be able to soon see the galaxies made up of the first stars, and thus understanding early star formation is critical as we observe parts of the universe never seen before.

In this work, I have studied the birth sites of Pop III stars using high resolution, cosmological simulations. I have found that the minimum mass threshold, the minimum threshold at which galaxies can form Pop III stars, is not affected by the instantaneous Lyman-Werner radiation background, and H2 self-shielding allows smaller mass halos to form Pop III stars. I have found that multiple Pop III stars can form in a single halo, and high mass halos can accumulate both young and old Pop III stars through hierarchical merging. I then focused on how neutron star mergers (NSMs) affect the second generation of stars by varying the explosion energy and the delay time, the time between NS binary system formation and r-process production, in a suite of zoom-in simulations of a single halo. I found that in general, a NSM leads to significant r-process enhancement in the second generation of stars. A high explosion energy leads to all enhanced r-process stars being highly enhanced, while a lower explosion energy leads to a higher mass fraction of stars being r-process enhanced, but not as many being highly r-process enhanced. When a NSM has a short delay time, there is a higher mass fraction of stars being r-process enhanced, but a smaller fraction being highly r-process enhanced. Finally, in collaboration with my research group, I have created a fitting pipeline to model the spectral energy distributions of photometric data of high redshift galaxies detected by JWST. We use scaling relations from high redshift cosmological simulations to better model galaxies in the early universe and determine their full star formation history. We also include other bright sources in the total SED, like an active galactic nuclei and binary stellar populations. We find that our fitting pipeline matches the photometry of high redshift galaxies very well, and conclude that models of the high redshift universe need to be further refined in order to accurately model this early environment.