Doctor of Philosophy
Metchev, Stanimir A.
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.
Summary for Lay Audience
In between stars and planets there is a class of astronomical object called brown dwarfs. Brown dwarfs share properties with both stars and planets; they are formed in the same way as stars, but have much lower masses, and they have thick, cloudy atmospheres similar to giant planets like Jupiter. The clouds in the atmospheres of brown dwarfs are made up of a variety of materials, and they tend to be patchy and varied in thickness. Storms, similar to Jupiter’s great red spot, can develop and evolve in the atmospheres of brown dwarfs.
The varying thickness and composition of the clouds means that different parts of a brown dwarf’s surface emit different amounts of light. As brown dwarfs rotate, different clouds and atmospheric structures will face the Earth at a given time, so the amount of light we measure will change over time. When we see a repeated pattern in the amount of light we measure, we know we are seeing the same surface features going in and out of view, and so we can measure how fast brown dwarfs are rotating. In the first half of this thesis, I present the discovery of the fastest-ever rotating brown dwarfs. They have rotation periods of approximately one hour, which is nearly ten times faster than Jupiter, and 24 times faster than the Earth. I explore the consequences of this fast rotation and put these speedy spinners in context with the rest of the known rotation periods for brown dwarfs.
The clouds on brown dwarfs are composed of many materials we are familiar with, like water, carbon monoxide, and methane. A key tool in understanding these materials, how they interact, and the physics governing all of this, are atmospheric models. In the second half of this thesis, I study these molecules and rigorously test the available models with one of the highest-resolution brown dwarf data sets ever observed. I draw conclusions about which of the currently-available models are the most reliable and describe how to measure fundamental properties of brown dwarfs, like their temperatures, with these models.
Tannock, Megan E., "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" (2021). Electronic Thesis and Dissertation Repository. 8354.
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