A&A 367, 218-235 (2001)
DOI: 10.1051/0004-6361:20000416
Variability in ultra cool dwarfs: Evidence for the evolution of surface features
C. A. L. Bailer-Jones and R. MundtMax-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
(Received 17 October 2000 / Accepted 30 November 2000 )
Abstract
We present photometric light curves for a sample of 21 ultra cool M
and L dwarfs in the field and in the young open clusters
Orionis and the Pleiades. The list of targets includes both
low mass hydrogen burning stars and brown dwarfs. Evidence for
variability with rms amplitudes (in the I band) of 0.01 to 0.055
magnitudes on timescales of 0.4 to 100 hours is discovered in half of
these objects. Power spectral analysis using the CLEAN
algorithm was performed to search for evidence of periodic
variability. Some objects show strong periodicities at around a few
hours, which could be due to rotational modulation of the light curve
by surface features. However, several objects do not have any
significant periodicities to explain their variability. The 
values of a similar population of objects makes it very likely that
our time sampling was sensitive to the expected range of rotation
periods, and simulations show that we would have detected these if
they were caused by long-lived surface features. We argue that this
absence of periodicity is due to the evolution of the brightness, and
presumably also the physical size, of surface features on timescales
of a few to a few tens of hours. This is supported in the case of
2M1145 for which two light curves have been obtained one year apart
and show no common periodicity. The surface features could plausibly
be photospheric dust clouds or magnetically-induced spots. The
recently observed decline in chromospheric activity for late type M
and L dwarfs hints towards the former explanation for at least our
later-type objects. Furthermore, our sample suggests that variability
is more common in objects later than M9, indicating that the
variability may be related to dust formation. One light curve shows a
brief, but significant, dip, which could be a short-lived feature or
possibly an eclipse by a companion.
Key words: methods: data analysis -- stars: atmospheres -- stars: low-mass, brown dwarfs -- stars: rotation -- stars: starspots -- stars: variables: others
Offprint request: C. A. L. Bailer-Jones, calj@mpia-hd.mpg.de
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