  We have conducted a photometric monitoring program of 3 field late-L brown
dwarfs looking for evidence of non-axisymmetric structure or temporal
variability in their photospheres. The observations were performed using
Spitzer/IRAC 4.5 and 8 micron bandpasses and were designed to cover at least
one rotational period of each object. One-sigma RMS (root mean squared)
uncertainties of less than 3 mmag at 4.5 micron and around 9 mmag at 8 micron
were achieved. Two out of the three objects studied exhibit some modulation in
their light curves at 4.5 micron - but not 8 micron - with periods of 7.4 hr
and 4.6 hr and peak-to-peak amplitudes of 10 mmag and 8 mmag. Although the lack
of detectable 8 micron variation suggests an instrumental origin for the
detected variations, the data may nevertheless still be consistent with
intrinsic variability since the shorter wavelength IRAC bandpasses probe more
deeply into late L dwarf atmospheres than the longer wavelengths. A cloud
feature occupying a small percentage (1-2 %) of the visible hemisphere could
account for the observed amplitude of variation. If, instead, the variability
is indeed instrumental in origin, then our non-variable L dwarfs could be
either completely covered with clouds or objects whose clouds are smaller and
uniformly distributed. Such scenarios would lead to very small photometric
variations. Followup IRAC photometry at 3.6 and 5.8 micron bandpasses should
distinguish between the two cases. In any event, the present observations
provide the most sensitive search to date for structure in the photospheres of
late-L dwarfs at mid-IR wavelengths, and our photometry provides stringent
upper limits to the extent to which the photospheres of these transition L
dwarfs are structured.
