Abstract

Massive stars are some of the most dynamic and influential objects in the Universe, driving galactic evolution through violent explosions and serving as progenitors to neutron stars and black holes. While the evolution of isolated massive stars already produces remarkable phenomenae, massive stars that interact with companions or evolve within extreme environments may produce even more extraordinary outcomes. In this dissertation, I investigate some exceptional circumstances in which a massive star might find itself. In a first investigation, I analyze the strong interactions between isolated pairs of stars in low-density environments. If a binary consisting of a massive star and a neutron star enters a common envelope, there are two possible outcomes: the formation of a short period binary (a potential gravitational wave source progenitor) or the merger of the massive star and the neutron star. If a merger occurs, a Thorne-Żytkow object (TŻO) may form, where the neutron star core is surrounded by a large diffuse envelope. In this study, I explore the properties of binaries that enter a common envelope with a massive star and a neutron star (e.g., possible TŻO progenitors), accounting for cases where previous stable mass transfer has rejuvenated the accretor. I characterize the population demographics, impact of rejuvenation, and calculate rates for several formation scenarios. In another investigation, I explore how stars will evolve in dense environments surrounding supermassive black holes. Active galactic nuclei (AGN) disks are expected to harbor large numbers of stars and black holes, which can be progenitors to gravitational wave events. I present my contributions to McFACTS, an open source population synthesis code designed to model the full AGN channel for LIGO-Virgo-KAGRA (LVK) detectable binary black hole mergers. As part of this work, I extended the code to model the evolution of stars and calculate their impact on the black hole population. I describe the core McFACTS code and the new stellar modules that allow for a more comprehensive picture of the AGN channel. I demonstrate how high rates of mass accretion create a population of stars which follow a unique evolutionary path and exert disproportionate influence on their environment.

Library of Congress Subject Headings

Supergiant stars--Observations; Supergiant stars--Evolution

Publication Date

12-2025

Document Type

Dissertation

Student Type

Graduate

Degree Name

Astrophysical Sciences and Technology (Ph.D.)

Department, Program, or Center

Physics and Astronomy, School of

College

College of Science

Advisor

Richard O’Shaughnessy

Advisor/Committee Member

George Thurston

Advisor/Committee Member

Andrew Robinson

Campus

RIT – Main Campus

Plan Codes

ASTP-PHD

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