Illuminating Brown Dwarfs: Exploring the Impact of Irradiation in Ultracool Atmospheres Through Phase-Resolved Spectroscopy

Abstract

Key questions regarding the impact of irradiation on hot Jupiter atmospheres remain unresolved, including cloud formation and evolution, the efficiency of day-to-night heat redistribution, and the effects and extent of thermal dissociation and recombination. Despite advancements in observations, hot Jupiters have historically been challenging to fully characterize, in part due to their relatively bright host stars. Irradiated brown dwarfs, especially those in short period ($<10$ hr) white dwarf-brown dwarf (WD+BD) binaries, serve as ideal analogs for tidally-locked hot Jupiter atmospheres, as they experience intense irradiation on one hemisphere and are governed by similar atmospheric processes. Consequently, the study of irradiated brown dwarfs offers a detailed understanding of the effects of irradiation on ultracool atmospheres, thereby informing our knowledge of hot Jupiters. Phase-resolved spectroscopy is a powerful method in this research, providing insights into the temporal and spatial variability of these atmospheres. In this dissertation, I present Hubble Space Telescope/Wide Field Camera 3 phase-resolved spectroscopy of three irradiated brown dwarfs in short-period WD+BD systems. This study estimates their global cloud coverage, assesses the presence or absence of thermal inversions, and predicts heat redistribution efficiencies. First, I present results on the NLTT5306 system, revealing a day$-$night temperature difference of less than 100~K and a complex wavelength-dependence in the phase curve amplitudes and offsets. Models fits to both day and night-side hemispheres indicated global coverage; the day-side matched irradiated models whereas the night-side matched non-irradiated models. Our simple irradiation redistribution model indicated efficient day-to-night heat redistribution and a moderately high Bond albedo. We discovered that the global temperature structure of NLTT5306B is primarily driven by its internal heat rather than external irradiation. The high internal heat prompted a reevaluation of the system age. Using updated GD1400A velocities from \textit{Gaia}, we derived a much younger system age than previously published, making the internal heat consistent with brown dwarf evolution models. I then present phase-resolved spectroscopy of white dwarf-brown dwarf binary SDSS\,1557. Analysis and modeling of the phase curves revealed significant rotational modulations with wavelength-dependent amplitudes. The extracted day-side spectrum showed a steep, nearly featureless slope, indicative of a dissociated atmosphere. The night-side spectrum was nearly undetected, providing an upper limit of 1600~K for the night-side brightness temperature. Our irradiation redistribution model suggested weak heat redistribution efficiency and indicated an inclination value higher than previously thought. Model fits to the night-side spectrum of SDSS 1557B suggest that this hemisphere is cloud-dominated, while the day-side spectrum is characterized by H- opacity and a temperature inversion. Finally, I present results on the analysis of the phase-resolved spectroscopy for the weakly irradiated brown dwarf GD1400B. Phase curves modeling and analysis revealed higher-order ZZ Ceti pulsation from the white dwarf and weak amplitude modulations, around 1\%, from GD1400B. The ZZ Ceti pulsations were shown to not have a significant impact on the spectral morphology, but they were still excluded from further analysis in order to preserve data-quality. The day and night brown dwarf spectra exhibited highly similar spectral features and overall total flux. Irradiated one-dimensional model fits to both hemispheres suggested global cloud coverage. Derived brightness temperatures showed consistently small temperature difference across the wavelength range, with a relatively hot night-side temperature ($T_{\rm{B,night}}=1950$~K). A simple irradiation redistribution model predicted highly efficient heat redistribution, although an estimate of radiative versus circulation timescales suggest that radiative cooling is highly efficient on the day-side, which may instead explain the ``efficient redistribution'' prediction. GD1400 is the final object the ``Dancing with the Dwarfs'' observational sample, consisting of six WD--BD binaries. At the end of this study, I present a comparative analysis across the six irradiated brown dwarfs and discover a possible irradiation threshold of $F_{\rm{irr}}\sim10^9$ ergs/s/cm$^2$, where internal heat dominates below the threshold and external irradiation dominates above. Together, these studies enhance our understanding of ultracool irradiated atmospheres, particularly regarding heat redistribution, cloud coverage, and the interaction between internal and external heat sources. They lay the groundwork for future investigations of white dwarf-brown dwarf binaries, which will bridge the gap between isolated brown dwarf and hot Jupiter atmospheres. Additionally, they provide critical data for refining models of atmospheric processes under extreme conditions.