Variability and periodicity of field M dwarfs revealed by multichannel monitoring

We present simultaneous, multiband monitoring of 19 field M dwarfs (spectral types M2-M9). Significant variability was found in seven objects in at least one out of the three channels I, R and G. Two objects, LHS370 (M5V) and 2M1707+64 (M9V), show periods of 5.9+-2.0 and 3.65+-0.1 hours respectively. On account of the agreement with the typical values of v*sin(i) published for M dwarfs (Mohanty & Basri 2003), we claim these to be the objects' rotation periods. In the case of 2M1707+64 we furthermore find evidence for the presence of magnetically-induced cool spots as the reason for the observed variability (at a temperature contrast of 4-8 percent, with a projected surface coverage factor of less than 0.075) and can rule out the scenario of dust clouds (as represented by the COND or DUSTY models). 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. This implies a change in the physical nature and/or extent of surface features when moving from M to L dwarfs.