Photometric I band light curves of 21 ultra cool M and L dwarfs are
presented. Variability with amplitudes of 0.01 to 0.055 magnitudes (RMS) with
typical timescales of an hour to several hours are discovered in half of these
objects. Periodic variability is discovered in a few cases, but interestingly
several variable objects show no significant periods, even though the
observations were almost certainly sensitive to the expected rotation periods.
It is argued that in these cases the variability is due to the evolution of the
surface features on timescales of a few hours. This is supported in the case of
2M1145 for which no common period is found in two separate light curves. It is
speculated that these features are photospheric dust clouds, with their
evolution possibly driven by rotation and turbulence. An alternative
possibility is magnetically-induced surface features. However, chromospheric
activity undergoes a sharp decrease between M7 and L1, whereas a greater
occurrence of variability is observed in objects later than M9, lending support
to the dust interpretation.Comment: To appear in "Ultracool Dwarf Stars" (Lecture Notes in Physics),
H.R.A. Jones, I. Steele (eds), Springer-Verlag, 2001. Also available from
http://www.mpia-hd.mpg.de/homes/calj/ultra.htm
