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 ms−2 (corresponding to masses of _ 15 to 70 MJupiter). 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 ms−2 or less, potentially providing a new
method for constraining L dwarf masses.
