Polarisation of very-low-mass stars and brown dwarfs

Abstract

Ultra-cool dwarfs of the L spectral type (Teff=1400-2200K) are known to havedusty atmospheres. Asymmetries of the dwarf surface may arise fromrotationally-induced flattening and dust-cloud coverage, and may result innon-zero linear polarisation through dust scattering.We aim to study the heterogeneity of ultra-cool dwarfs' atmospheres and thegrain-size effects on the polarisation degree in a sample of nine late M, L andearly T dwarfs.We obtain linear polarimetric imaging measurements using FORS1 at the VeryLarge Telescope, in the Bessel I filter, and for a subset in the Bessel R andthe Gunn z filters.We measure a polarisation degree of (0.31+/-0.06)% for LHS102BC. We fail todetect linear polarisation in the rest of our sample, with upper-limits on thepolarisation degree of each object of 0.09% to 0.76% (95% of confidence level).For those targets we do not find evidence of large-scale cloud horizontalstructure in our data. Our results decrease the fraction of ultra-cool dwarfswith detected linear polarisation to (23+10-6)% (1-sigma errors).For three brown dwarfs, our observations indicate polarisation degreesdifferent (at the 3-sigma level) than previously reported, giving hints ofpossible variations.Our results fail to correlate with the current model predictions forultra-cool dwarf polarisation for a flattening-induced polarisation, or withthe variability studies for a polarisation induced by an hetereneous cloudcover. This stresses the intricacy of each of those tasks, but may as wellproceed from complex and dynamic atmospheric processes.