Precision photometric monitoring of very low mass Sigma Orionis cluster members: variability and rotation at a few Myr

We present high-precision photometry on 107 variable low-mass stars and brown dwarfs in the ~3 Myr Sigma Orionis open cluster. We have carried out I-band photometric monitoring within two fields, encompassing 153 confirmed or candidate members of the low-mass cluster population, from 0.02 to 0.5 M_\odot. We are sensitive to brightness changes on time scales from 10 minutes to two weeks with amplitudes as low as 0.004 magnitudes, and find variability on these time scales in nearly 70% of cluster members. We identify both periodic and aperiodic modes of variability, as well as semi-periodic rapid fading events that are not accounted for by the standard explanations of rotational modulation of surface features or accretion. We have incorporated both optical and infrared color data to uncover trends in variability with mass and circumstellar disks. While the data confirm that the lowest-mass objects (M<0.2M_\odot) rotate more rapidly than the 0.2-0.5 M_\odot members, they do not support a direct connection between rotation rate and the presence of a disk. Finally, we speculate on the origin of irregular variability in cluster members with no evidence for disks or accretion.