Optical linear polarimetry of ultra cool dwarfs

We present optical linear polarimetry of 8 ultra cool field dwarfs, with spectral types ranging from M9 to L8. The linear polarisation, P, of each dwarf we measured is P< 0:2%. Three dwarfs have polarisations compatible with zero, two are marginal detections, and three have significant polarisation. Due to their small distance, an insterstellar origin for the detected polarisation can be safely ruled out. Our detections confirm that dust is present in the atmosphere of these brown dwarfs and that the scattering geometry is not symmetric. Possibilities for asymmetry include the dwarfs rotating rapidly and being oblate, or the cloud coverage in the atmosphere being inhomogenous.