Brown Dwarf Atmospheres at High Cadence and Spectral Resolution: A Speed Limit on Brown Dwarf Rotation and a Spectroscopic Atlas of a 1050 K Atmosphere
Abstract
Brown dwarfs are sub-stellar objects that form like stars but are not sufficiently massive to sustain hydrogen fusion in their cores. Characterized by cool, molecule-rich atmospheres, brown dwarfs demonstrate great diversity in spectroscopic appearance and share many properties with giant exoplanets. In this thesis I present two investigations: the first is a detailed photometric and spectroscopic study of the three most rapidly rotating brown dwarfs. The second examines a spectrum of a cool brown dwarf at unprecedented spectral resolution and signal-to-noise ratio to study the accuracy of theoretical model photospheres.
Photometric monitoring of brown dwarfs has revealed that periodic variability is common and that brown dwarf atmospheres are composed of patchy, multi-layer clouds of varying thicknesses and compositions. In my first paper, I present the discovery of rapid photometric variability in three brown dwarfs from long-duration photometric monitoring with the Spitzer Space Telescope. Using moderate-resolution infrared spectroscopy I find a large degree of rotational broadening in each of these brown dwarfs, confirming that the rapid variability is due to fast rotation. These three brown dwarfs have the shortest rotation periods ever measured, between 1.08 and 1.23 hours. When put in context with the entire sample of brown dwarfs with known rotation periods, the clustering near the short-period end suggests that brown dwarfs are unlikely to spin much faster than once an hour.
In my second paper, I study the atmospheric composition of a cold 1050 +- 50 K (T6-type) brown dwarf. Even the most up-to-date theoretical model photospheres do not completely reproduce observed spectroscopic features in cold brown dwarfs, limiting our ability to constrain their fundamental properties. I compare the observed data to these models to assess their accuracy and completeness. I draw conclusions about which models are the most reliable and which spectroscopic regions should be used to estimate physical parameters of cold brown dwarfs and, by extension, exoplanets. Additionally, I present the first unambiguous detections of hydrogen sulfide in an extra-solar atmosphere. These data comprise the most detailed atlas of spectroscopic lines in a cold brown dwarf available to date.