UCLA

Brown dwarfs are a unique laboratory for understanding atmospheric physics in planetary-mass objects. Though they form similarly to stars, brown dwarfs have cooler temperatures and lower masses, making their atmospheres often more analogous to those of giant planets. While significant progress has been made in characterizing their bulk properties and lower atmospheres, the upper atmospheres of brown dwarfs remain largely unexplored. Upper atmospheric regions are critical because they mediate interactions between the space environment, atmosphere, and planetary interior, absorbing high-energy radiation externally, while internally coupling with magnetic fields and exchanging energy with the lower atmosphere through radiative transfer and atmospheric waves.

This dissertation investigates the properties and evolution of brown dwarf atmospheres, with a focus on energetic chemistry in the upper atmosphere, using a combination of new observations, theoretical modeling, and instrumentation development. I begin with new observations of the benchmark brown dwarf HD 33632 Ab, examining the importance of clouds, disequilibrium chemistry, and composition in comparing near-infrared spectra with evolutionary models. Next, I present the first high-resolution spectroscopic search for upper atmospheric H3+ emission in the near-infrared L band, placing upper limits on its emission and probing potential differences in auroral chemistry between brown dwarfs and giant planets. I then model the photochemical and thermal response of brown dwarf upper atmospheres to energetic flares from active M dwarf companions, predicting observable spectral signatures. Finally, I describe image simulations in support of the upcoming High-resolution Infrared Spectrograph for Exoplanet Characterization (HISPEC) instrument at Keck Observatory, which will enable next-generation high-resolution studies of exoplanet and brown dwarf atmospheres. Collectively, these studies underscore the complexity and current gaps in our understanding of brown dwarf upper atmospheres and help lay the foundation for future advances in the field.