Understanding the Depths of Brown Dwarfs and Giant Exoplanets: Modelling Substellar Atmospheres

Abstract

The absence or lack of steady hydrogen fusion in the cores of brown dwarfs means these objects cool over time by radiating away their internal thermal energy. The rate at which these objects cool is regulated by the atmosphere, which imprints its complex and changing chemical composition of molecules and condensates onto the emitted radiation. A reliable model of the atmosphere and its evolution over time therefore lies at the core of our understanding of brown dwarfs and substellar objects. Over the last decade the WISE mission has uncovered the coolest spectral type known as the Y dwarfs. These objects have effective temperatures a few times greater than Jupiter, and thus provide excellent analogs for Jovian-like worlds outside of our solar system. Accurate and reliable atmosphere and evolution models are important for placing mass and age constraints on these newly discovered objects and understanding the rich chemistry and physics taking place in their atmospheres. In this thesis, I present a new set of solar metallicity atmosphere and evolutionary models for very cool brown dwarfs and self-luminous giant exoplanets, which is termed ATMO 2020. Atmosphere models are generated with the state-of-the-art 1D radiative-convective equilibrium code ATMO, and are used as surface boundary conditions to calculate the interior structure and evolution of 0.0005-0.075 solar mass objects. These models include several key improvements to the input physics used in previous models available in the literature. First, the use of a new H-He equation of state including ab initio quantum molecular dynamics calculations has raised the mass by ~1-2% at the stellar-substellar boundary and has altered the cooling tracks around the hydrogen and deuterium burning minimum masses. A second key improvement concerns updated molecular opacities in our atmosphere model ATMO, which now contains significantly more line transitions required to accurately capture the opacity in these hot atmospheres. This leads to warmer atmospheric temperature structures, further changing the cooling curves and predicted emission spectra of substellar objects. I present significant improvement for the treatment of the collisionally broadened potassium resonance doublet, and highlight the importance of these lines in shaping the red-optical and near-infrared spectrum of brown dwarfs. This is highlighted through improved comparisons to the observed spectra of benchmark objects. I generate three different grids of model simulations, one using equilibrium chemistry and two using non-equilibrium chemistry due to vertical mixing, all three computed self-consistently with the pressure-temperature structure of the atmosphere. I show the impact of vertical mixing on emission spectra and in colour-magnitude diagrams, and highlight wavelength regions which can be used of infer the strength of vertical mixing in cool brown dwarfs.