The Circumstellar Disks and Binary Companions of Be Stars
Drew Chojnowski, NMSU
Tremendous progress has been made over the past two decades toward understanding Be stars, but a number of key aspects of them remain enigmatic. The unsolved mysteries include identification of the mechanism responsible for disk formation, the reason this mechanism occasionally turns off or on unexpectedly, the source of viscosity in the circumstellar disks, and the cause of slowly precessing density perturbations in the disks of many or most Be stars. On a deeper level, the origin of Be stars’ near-critical rotation is unknown, with one possible explanation being spin-up due to interaction with a binary companion. A better understanding of these stars is needed, with a particular focus on high-mass binaries being warranted in the age of gravitational wave astronomy. In this dissertation, I will extend the knowledge and understanding of Be stars through a series of three projects. First, I will present and describe the largest ever homogeneous, spectroscopic sample of Be stars to date. I will then focus on investigation of a rare class of Be stars found in binary systems with hot, low mass companions. The second project will present detailed characterization and modeling of HD~55606, a newly discovered member of this class. Finally, I will discuss the results of spectroscopic monitoring of seven newly discovered systems and establish or place limits on the orbital parameters of the binary components.
Metal Absorption in the Circumgalactic Medium During the Epoch of Reionization
Caitlin Doughty, NMSU
The characteristics of metal absorption arising from the circumgalactic medium of galaxies have been demonstrated to be related to conditions in the galaxy which sourced them, as well as to the ambient ultraviolet background. I propose a three- pronged thesis in order to better understand and utilize these relationships. First, I will explore whether the spectral energy distributions of binary stars, incorporated into a custom version of GADGET-3, can explain the discrepancy between observed and simulated absorber statistics. Second, I will study the relationship between neu- tral oxygen absorbers and the neutral hydrogen fraction in simulated quasar sight- lines and relate the results to observations of neutral oxygen at z ≥ 4.0. Third, I will study the relationships between the emissive properties of galaxies, stemming from their nebular gas, and the metal absorbers which they source. Taken as a whole, this thesis will improve the ability of cosmological simulations to reproduce realistic metal absorption, probe the local progress and topology of reionization, and under- stand what emissive galaxy traits we expect at z > 5 based on observations of metal absorbers.