Observed Polarization of Brown Dwarfs Suggests Low Surface Gravity

TL;DR

This paper suggests that the polarization observed in some L dwarfs is likely caused by their oblate shape due to rotation, and low surface gravity enhances polarization, offering a new way to estimate their masses.

Contribution

It introduces a model combining multiple scattering and atmospheric analysis to link polarization with low surface gravity in L dwarfs, aiding mass estimation.

Findings

Polarization indicates low surface gravity (~300 m/s^2 or less).

Oblate shape from rotation likely causes observed polarization.

Provides a new method for constraining L dwarf masses.

Abstract

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.