Name/Affiliation: Tyler D. Robinson (NASA Ames Research Center)
Modeling Sources of Variability in Brown Dwarf Spectra
A number of brown dwarfs are known to be variable with observed amplitudes as large as 10-30% at some wavelengths. A combination of dynamical effects, temporally- and spatially-varying clouds, and associated atmospheric temperature fluctuations is likely responsible for the observed variability. Using a newly-developed one-dimensional, time-stepping model of atmospheric thermal structure, we explore the evolution of thermal perturbations in a T-dwarf atmosphere. We demonstrate that thermal perturbations occurring deep in the atmosphere can be communicated to the upper atmosphere through radiative heating via the windows in near-infrared water opacity. The atmospheric response timescale depends on where a thermal perturbation is introduced. For certain periodic perturbations, we show that the emission spectrum can have complex, time- and wavelength-dependent behaviors, including phase shifts in times of maximum flux observed at different wavelengths. Since different wavelengths probe different regions of the atmosphere, observed variations track, in part, a complex, wavelength-dependent set of radiative exchanges happening between different atmospheric levels. We conclude that thermal fluctuations should be considered as an important contributor to brown dwarf spectral variability, and discuss the potential feedbacks between these fluctuations and clouds.