Protostars and Planets VI, Heidelberg, July 15-20, 2013

Poster 2G023

Probing the Heterogeneous Cloud Structure of Variable Brown Dwarfs with HST

Buenzli, Esther (MPIA Heidelberg & University of Arizona)
Apai, Dániel (University of Arizona)
Radigan, Jacqueline (Space Telescope Science Institute)
Morley, Caroline (University of California, Santa Cruz)
Burrows, Adam (Princeton University)
Flateau, Davin (University of Arizona)
Showman, Adam (University of Arizona)
Marley, Mark (NASA Ames)
Reid, Iain Neill (Space Telescope Science Institute)
Lewis, Nikole (Massachusets Institute of Technology)
Jayawardhana, Ray (University of Toronto)

With masses between those of stars and planets and temperatures similar to young giant planets, brown dwarfs offer a unique opportunity to study ultracool atmospheres without the inconvenience of a bright host star. Cloud structure and evolution play a very important role in these atmospheres. In particular, the transition from cloudy L-type to (mostly) clear T-type dwarfs at ∼1200-1300 K is not yet well understood. Favored models include cloud thinning due to particle growth and rain-out or the formation of holes. Recent discoveries of brown dwarfs with significant flux variability in the near-infrared have indicated the presence of heterogeneous cloud structure and evolving weather patterns. We conducted the first time-resolved spectroscopic study of 3 variable brown dwarfs with HST/WFC3 covering their full rotation periods. The high-quality spectral time series from 1.1-1.7 microns yielded surprising results. Two early T dwarfs show evidence for a mixture of two cloud components (thin and thick clouds) but an absence of deep cloud holes. Our third object is a T6 dwarf beyond the L/T transition for which thin condensate clouds are needed to fit the average spectrum. For this object we obtained both HST spectroscopy and simultaneous Spitzer photometry at 4.5 microns. The spectral variability of this cooler brown dwarf is significantly different from the L/T transition objects. Most intriguingly, the light curves at different wavelengths are not in phase, and there is a correlation between the phase shift and the pressure probed at a given wavelength, indicating complex vertical structure. Our study demonstrates the power of time domain observations of ultracool atmospheres that will soon be extended to additional objects and longer time scales.

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