Rotation and accretion of very low mass objects in the SigmaOri cluster

We report on two photometric monitoring campaigns of Very Low Mass (VLM) objects in the young open cluster around SigmaOrionis. Our targets were pre-selected with multi-filter photometry in a field of 0.36 sqdeg. For 23 of these objects, spanning a mass range from 0.03 to 0.7 MS, we detect periodic variability. Of these, 16 exhibit low-level variability, with amplitudes of less than 0.2 mag in the I-band, which is mostly well-approximated by a sine wave. These periodicities are probably caused by photospheric spots co-rotating with the objects. In contrast, the remaining variable targets show high-level variability with amplitudes ranging from 0.25 to 1.1 mag, consisting of a periodic light variation onto which short-term fluctuations are superimposed. This variability pattern is very similar to the photometric behaviour of solar-mass, classical T Tauri stars. Low-resolution spectra of a few of these objects reveal strong Halpha and Ca-triplet emission, indicative of ongoing accretion processes. This suggests that 5-7% of our targets still possess a circumstellar disk. In combination with previous results for younger objects, this translates into a disk lifetime of 3-4 Myr, significantly shorter than for solar mass stars. The highly variable objects rotate on average slower than the low-amplitude variables, which is expected in terms of a disk-locking scenario. There is a trend towards faster rotation with decreasing mass, which might be caused by shortening of the disk lifetimes or attenuation of magnetic fields.