Explaining the Observed Polarization from Brown Dwarfs by Single Dust Scattering

Abstract

Recent observation of linear optical polarization from brown dwarfs confirms the dust hypothesis in the atmospheres of brown dwarfs with effective temperature higher than 1400 K. The observed polarization could arise because of dust scattering in the rotation induced oblate photosphere or because of the scattering by nonspherical grains in the spherical atmosphere or by the anisotropic distribution of dust clouds. Assuming single scattering by spherical grains in a slightly oblate photosphere consistent with the projected rotational velocity, the observed optical linear polarization is modeled by taking grains of different sizes located at different pressure height and of different number density. Minimum possible oblateness of the object due to rotation is considered in order to constrain the grain size. It is shown that the observed polarization from the L dwarfs 2MASSW J0036+1821 and DENIS-P J0255-4700 can be well explained by several sets of dust parameters and with the minimum possible oblateness. Models for the observed polarization constrain the maximum size of grains. It is emphasized that future observation of polarization at the blue region will further constrain the grain size.