Polarization of ultra-cool dwarfs

Ultra-cool dwarfs with effective temperatures between approximate to 1400 K and approximate to 2200 K are known to have dusty atmospheres. Asymmetries of the dwarf surface may arise from rotationally-induced flattening and dust-cloud coverage, and may result in non-zero linear polarisation through dust scattering. Here we present our linear polarimetric imaging measurements using FORS 1 at the Very Large Telescope, in Paranal, in the Bessel I filter, and for a subset of our sample in the Bessel R and the Gunn z filters. We report a polarisation degree of (-0.31 +/- +0.07)% for LHS102BC. We fail to detect linear polarisation in the rest of our sample, with upper-limits on the polarisation degree of each object of 0.09% to 0.76% (95% of confidence level), depending on the targets and the bands. For six targets, our data indicate polarisation degrees smaller (at the 2-sigma level) than previously reported.