Jacqueline Radigan, Ray Jayawardhana, David Lafrenière, Etienne Artigau, Mark Marley, Didier Saumon
We present multiple-epoch photometric monitoring in the $J$, $H$, and $K_s$
bands of the T1.5 dwarf 2MASS J21392676+0220226 (2M2139), revealing persistent,
periodic ($P=7.72\pm$0.05 hr) variability with a peak-to-peak amplitude as high
as 26% in the $J$-band. The light curve shape varies on a timescale of days,
suggesting that evolving atmospheric cloud features are responsible. Using
interpolations between model atmospheres with differing cloud thicknesses to
represent a heterogeneous surface, we find that the multi-wavelength variations
and the near-infrared spectrum of 2M2139 can be reproduced by either (1)cool,
thick cloud features sitting above a thinner cloud layer, or (2)warm regions of
low condensate opacity in an otherwise cloudy atmosphere, possibly indicating
the presence of holes or breaks in the cloud layer. We find that temperature
contrasts between thick and thin cloud patches must be greater than 175 K and
as high as 425 K. We also consider whether the observed variability could arise
from an interacting binary system, but this scenario is ruled out. 2M2139 joins
the T2.5 dwarf SIMP0136 discovered by Artigau and coworkers as the second L/T
transition brown dwarf to display large-amplitude variability on rotational
timescales, suggesting that the fragmentation of dust clouds at the L/T
transition may contribute to the abrupt decline in condensate opacity and
$J$-band brightening observed to occur over this regime.
View original:
http://arxiv.org/abs/1201.3403
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