UC Santa Cruz

The spectral resolution of instruments used to characterize substellar atmospheres has greatly increased over the past decade. As we more frequently observe exoplanet and brown dwarf atmospheres at higher spectral resolution, more work is needed to assess both what new information is contained in these improved observations as well as how our current modeling tools fall short in accurately reproducing these spectra. My dissertation has examined this question from multiple angles to ultimately prepare the field to better understand substellar atmospheres through the better quality spectra we will receive from the JWST and the upcoming ELTs. First, I present how high-resolution cross-correlation spectroscopy (R $\sim$ 25,000 to 100,000) will allow us to probe the regions in the atmospheres of sub-Neptune exoplanets above the clouds or hazes which obscure molecular features in observations at low spectral resolution. Using theoretical models of high-resolution observations for a typical hazy sub-Neptune, we calculate the signal-to-noise of these spectra required to robustly detect a host of molecules as a function of spectral resolution and wavelength coverage to aid in planning future observations and instruments.

Next, I present two projects focused on adapting atmospheric retrieval methods for medium-resolution spectra of brown dwarfs. I first describe applying a GPU-version of the CHIMERA retrieval framework to a high signal-to-noise, medium-resolution (R$\sim$6000) FIRE spectrum of a T9 dwarf from 0.85-2.5 $\mu$m. At 60$\times$ higher spectral resolution than previous brown dwarf retrievals, a number of novel challenges arise, which I explore. I show that compared to retrieval results from a R$\sim$100 spectrum of the same object, constraints on atmospheric abundances improve by an order of magnitude or more with increased spectral resolution.

Finally, I apply lessons learned from this project to JWST NIRSpec/G395H (R$\sim$2700, 2.87 - 5.14 $\mu$m) observations of a T8 dwarf, presenting the first retrieval analysis taking full advantage of the maximum spectral resolution available with NIRSpec. I obtain precise ($\sim$ 0.02 dex) abundance constraints for a number of species, which indicate shortcomings in our understanding of disequilibrium chemistry in brown dwarf atmospheres. I also present the measured $^{12}$CO/$^{13}$CO ratio for this brown dwarf, making it the fourth and coldest ($\sim$ 760 K) extrasolar object with such a measurement. Together, these projects illustrate the power of high-quality, medium-to-high resolution spectra to precisely constrain atmospheric properties, furthering our understanding of the formation and atmospheres of substellar objects.