Polarization of ultra-cool dwarfs

摘要

Ultra-cool dwarfs with effective temperatures between≈1400K and ≈2200 K are knownto have dusty atmospheres. Asymmetries of the dwarf surface may arise from rotationally-inducedflattening and dust-cloud coverage, and may result in non-zero linear polarisation through dustscattering. Here we present our linear polarimetric imaging measurements using FORS 1 at the VeryLarge Telescope, in Paranal, in the Bessel I filter, and for a subset of our sample in the Bessel R andthe Gunn z filters. We report a polarisation degree of (-0.31 + +0.07)% for LHS102BC. We fail todetect linear polarisation in the rest of our sample, with upper-limits on the polarisation degree ofeach 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-σ level) than previouslyreported.