The Complex Clouds and Chemistry of Directly Imaged Exoplanets and Brown Dwarfs

Abstract: Atmospheric retrieval and forward modeling are the only methods through which properties of substellar atmospheres can be determined since in situ measurements are impossible at astronomical distances. Constraining bulk properties like mass, radius, metallicity, and abundance ratios of carbon, oxygen, and isotopologues are required to understand the formation and evolution of these bodies, but this characterization faces many challenges. Modeling assumptions are necessary to compute synthetic spectra in a reasonable timeframe, but incorrect assumptions in forward modeling results in poor fits to the data, and incorrect assumptions in retrievals lead to inaccurate interpretation of results and the incorrect inference of bulk properties. The near- and mid- infrared spectra of hot objects (1000 K < Teff < 2100 K) are heavily impacted by the rainout of refractory elements into clouds and the near- and mid- infrared spectra of cold objects (Teff < 600 K) are difficult to interpret due to disequilibrium chemistry and vertical mixing in addition to the expected rainout of strong absorbers like water, potassium, and ammonia. Here I will present the work done in my dissertation to understand these complex worlds. I will present a retrieval analysis that demonstrates how assumptions commonly used in the modeling of hot, cloudy substellar objects bias retrieved bulk properties. I will also present a forward modeling analysis that precisely constrains silicate cloud properties from JWST spectra of YSES-1 c, a young directly-imaged exoplanet. Moving down in temperatures, I will present atmospheric analyses of some of the coldest substellar objects observed with JWST to demonstrate how combining retrievals and forward modeling can help us determine how disequilibrium chemistry and vertical mixing can explain the spectral diversity of cold worlds. Finally, I will present the first detection of deuterium (a powerful formation tracer) in an atmosphere outside of the solar system.