Photopolarimetric Characteristics of Brown Dwarfs. I. Uniform Cloud Decks

Abstract

This work is a theoretical exploration for facilitating the interpretation of polarimetric observations in terms of cloudiness, rotational velocities and effective temperatures of brown dwarfs (BDs). An envelope of scatterers like free-electrons, atoms/molecules, or haze/clouds affects the Stokes-vector of radiation emitted by oblate bodies. Due to high rotation rates, BDs can be considerably oblate. We present a conics-based radiative transfer (RT) scheme for computing the disc-resolved and disc-integrated polarized emission of an oblate BD or extrasolar giant planet (EGP) bearing homogenous or patchy clouds. Assuming a uniform grey atmosphere, we theoretically examine the photopolarimetric sensitivity to its scattering properties like cloud optical thickness and grain-size, concurrently with BD properties, like oblateness, inclination and effective temperature, which are all treated as free parameters. Additionally, we examine the potential effects of gravitational darkening (GD), revealing that it could significantly amplify disc-integrated polarization. GD imparts a non-linear inverse temperature-dependence to the resulting polarization. Photopolarimetric observations are sensitive to oblateness and inclination. The degree-of-polarization (DoP) increases in response to both, making it potentially useful for assessing the spatial orientation of the BD. Under our model assumptions, increasing droplet size in optically thick clouds causes a blue-ward shift in near-infrared (NIR) colors of BDs --- interesting in view of the observed J-K brightening in L/T transition. For large cloud grains, polarization decreases sharply, while transmitted intensity shows a steady increase. BD polarization is thus a potential indicator not only of the presence of clouds but also provides information on cloud grain size.