Self-Luminous Worlds with Exotic Clouds: Characterizing Clouds in Brown Dwarf Atmospheres
AuthorLew, Wei Peng
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PublisherThe University of Arizona.
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AbstractThe properties of clouds is perhaps one of the most crucial factors for correctly interpreting the observed emission and transmission spectra of planetary atmospheres with a wide range of temperatures, gravities, metallicities, and ages. The large number of available high-precision spectra of ultracool brown dwarfs provide an ideal testbed to identify the key physical and chemical atmospheric processes. My research aims to tackle a key missing piece in our current understanding of brown dwarf atmospheres -- What are the possible heterogeneous cloud and atmospheric structure in brown dwarf atmospheres? To answer the question, I utilize state-of-the-art Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) time-series spectroscopy to study brown dwarf atmospheres. Through high-precision time-series spectroscopy, we can simultaneously probe different atmospheric depths and constrain the heterogeneous cloud structure.In my first paper (Chapter 2), I report the discovery of a high rotational modulation amplitude of unusually red L dwarf WISEP J004701.06+680352.1 (WISEPJ0047). I use a simple sub-micron particle layer to explain the wavelength-dependence in the 1.1--1.7µm spectral variability. In my second paper (Chapter 3), I use a self-consistent heterogeneous cloud model that includes disequilibrium chemistry to explain the time-averaged spectra and spectral variability of WISEPJ0047. The modeling results suggest that a cloud thickness variation of around one pressure scale height can explain the observed spectral variability. In my third paper, I confirm the modulation amplitude of a planetary-mass companion GU Piscium b, which is a mid-T dwarf, and measure the spectral variability over 6 HST orbits. I measure the J-H color variations of GU Psc b and of other 11 brown dwarfs which have been observed in the same observational mode. I infer the weak J-H color modulations among the 12 mid-L to late-T dwarfs as evidence of gray cloud opacity being the dominant source of the observed rotational modulation in brown dwarf atmospheres. In Chapter 4, I present the spectroscopic phase curve of a strongly irradiated brown dwarf orbiting a white dwarf. I conclude that cloudless atmospheric models can explain the strong wavelength-dependent day-night spectral variation. Based on the atmospheric models, I construct the pressure-dependent day-night temperature contrast of the irradiated brown dwarf in the 2-80 bars range.
Degree ProgramGraduate College