Self-Luminous Worlds with Exotic Clouds: Characterizing Clouds in Brown Dwarf Atmospheres

Abstract

The 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.