Variability and periodicity of field M dwarfs revealed by multichannel monitoring
Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany e-mail: calj@mpia.de
Received:
5
September
2005
Accepted:
15
November
2005
We present simultaneous, multiband photometric monitoring
of 19 field dwarfs covering most of the M spectral sequence
(M2–M9). Significant variability was found in seven objects in at
least one out of the three channels I, R and G. Periodic
variability was tested with a CLEAN power spectral analysis. Two
objects, LHS370 (M5V) and 2M1707+64 (M9V), show periods of 
and 
h respectively. On account of the agreement
with the typical values of 
published for M dwarfs
(Mohanty & Basri 2003, ApJ, 583, 451),
we claim these to be the objects' rotation
periods. Three further objects show possible periods of a few
hours. Comparing the variability amplitude in each channel with
predictions based on the synthetic spectra of Allard et al. (2001, ApJ, 556, 357),
we investigated the source of variability in LHS370 and 2M1707+64. For
the latter, we find evidence for the presence of magnetically-induced
cool spots at a temperature contrast of 
, with a projected
surface coverage factor of less than 0.075.
Moreover, we can rule out dust clouds (as represented by the COND or DUSTY
models) as the cause of the variability. No conclusion can be drawn in
the case of LHS370. Comparing the frequency of occurrence of
variability in this and various L dwarf samples published
over the past few years, we find that variability is more common in
field L dwarfs than in field M dwarfs (for amplitudes larger than 
on timescales of 0.5 to 20 h). Using the homogeneous
data sets of this work and Bailer-Jones & Mundt (2001, A&A, 367, 218),
we find fractions
of variable objects of 
among field M dwarfs and
among field L dwarfs (and 
, 
respectively if we take into account a larger yet more
inhomogeneous sample). This is marginally significant (
deviation) and implies a change in the physical nature and/or extent
of surface features when moving from M to L dwarfs.
Key words: methods: data analysis / techniques: photometric / stars: late-type / stars: rotation / starspots / stars: variables: general
© ESO, 2006