Variability and periodicity of field M dwarfs revealed by multichannel monitoring

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 5.9 +/- 2.0 and 3.65 +/- 0.1 h respectively. On account of the agreement with the typical values of v sin i 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 4-8%, 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 0.005 mag 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 0.21 +/- 0.11 among field M dwarfs and 0.70 +/- 0.26 among field L dwarfs (and 0.29 +/- 0.13, 0.48 +/- 0.12 respectively if we take into account a larger yet more inhomogeneous sample). This is marginally significant (2 sigma deviation) and implies a change in the physical nature and/or extent of surface features when moving from M to L dwarfs.