Observed Polarization of Brown Dwarfs Suggests Low Surface Gravity

Details Observed Polarization of Brown Dwarfs Suggests Low Surface Gravity Observed Polarization of Brown Dwarfs Suggests Low Surface Gravity

2010-09-11

arxiv.org/abs/1009.2145v1

Astronomy and Astrophysics

Astrophysics

Solar and Stellar Astrophysics

14 pages (AASTEX) including 3 eps figures. Accepted for publication in The Astrophysical Journal Letters

Scientific paper

Light scattering by atmospheric dust particles is responsible for the polarization observed in some L dwarfs. Whether this polarization arises from an inhomogeneous distribution of dust across the disk or an oblate shape induced by rotation remains unclear. Here we argue that the latter case is plausible and, for many L dwarfs, the more likely one. Furthermore evolutionary models of mature field L dwarfs predict surface gravities ranging from about 200 to 2500 m/s^2 (corresponding to masses of about 15 to 70 times of Jupiter mass). Yet comparison of observed spectra to available synthetic spectra often does not permit more precise determination of the surface gravity of individual field L dwarfs, leading to important uncertainties in their properties. Since rotationally-induced non-sphericity, which gives rise to non-zero disk-integrated polarization, is more pronounced at lower gravities, polarization is a promising low gravity indicator. Here we combine a rigorous multiple scattering analysis with a self-consistent cloudy atmospheric model and observationally inferred rotational velocities and find that the observed optical polarization can be explained if the surface gravity of the polarized objects is about 300 m/s^2 or less, potentially providing a new method for constraining L dwarf masses.

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