Dissertations, Theses, and Capstone Projects

Date of Degree

9-2025

Document Type

Doctoral Dissertation

Degree Name

Doctor of Philosophy

Program

Physics

Advisor

Jacqueline K. Faherty

Committee Members

Kelle L. Cruz

Channon Visscher

Ben Burningham

K. E. Saavik Ford

Subject Categories

Stars, Interstellar Medium and the Galaxy

Keywords

Astronomy, Brown Dwarf, Giant Exoplanets

Abstract

In the era of the James Webb Space Telescope (JWST), it is now possible to examine substellar mass atmospheres in unparalleled detail and precision. However, unraveling the complicated thermodynamics and chemistry in these atmospheres remains a challenge. Atmospheric retrievals are a leading data-driven technique capable of recovering an object’s fundamental parameters such as mass, radius, effective temperature, gravity, cloud structure and composition and chemical abundances. However, in directly imaged or isolated worlds, constraining and/or interpreting these fundamental parameters can be difficult, particularly as retrieval models try to parameterize the overall impact of clouds.

For my dissertation, I turned to main sequence star – brown dwarf companion systems where the main sequence host can anchor metallicity, age, and abundance measurements for its substellar companion. I present results from conducting theoretical chemical analyses on a sample of benchmark companion brown dwarfs as well as solar neighborhood F, G, K-type stars to provide a global overview of the types of chemistry expected in nearby objects. Via mass balance and stoichiometric calculations, I predict the amount of oxygen that is being sequestered into refractory condensates in substellar atmospheres based on examined abundances of Mg, Si, Ca, Al, Ti, V and O. Additionally, I use solar neighborhood Mg/Si ratios to predict silicate condensation species (i.e. enstatite (MgSiO_3), forsterite (Mg_2SiO_4), and/or quartz (SiO_2) in a given companion. I extend this work beyond the brown dwarf regime to aid our understanding of giant exoplanet atmospheric chemistry by grounding models with their host star data. This work is crucial to link theory with observational data in order to test and calibrate evolutionary models and clarify our understanding of substellar formation.

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