Photopolarimetric characteristics of brown dwarfs bearing uniform cloud decks

Photopolarimetric characteristics of brown dwarfs bearing uniform cloud decks 

Authors: 

Sanghavi et al 

Abstract:

It has long been known that an envelope of scattering particles like free electrons, atoms and molecules, or particulate aggregates like haze or cloud grains affect the intensity and polarization of radiation emitted by a rotating body (Chandrasekhar 1946; Harrington and Collins 1968, Sengupta and Marley 2010, Marley and Sengupta 2011, de Kok et al. 2011). Due to their high rotation rates, brown dwarfs (BDs) are expected to be considerably oblate. We present a conics-based radiative transfer scheme for computing the disc-resolved and disc-integrated polarized emission of an oblate body. Using this capability, we examine the photopolarimetric signal of BDs as a function of the scattering properties of its atmosphere like cloud optical thickness and cloud grain size as well as properties specific to the BD such as its oblateness and the orientation of its rotation axis relative to the observer. The polarizing effect of temperature inhomogeneity caused by gravity-darkening is considered distinctly from the effect of oblateness, revealing that resulting temperature gradients cause intensity differences that can amplify the disc-integrated polarization by a factor of 2. Our examination of the properties of scatterers suggests that the contested relative brightening in the J-band for cooler BDs in the L/T-transition can partly be explained by thick clouds bearing larger-sized grains. Grain-size affects both the intensity and polarization of emitted radiation - as grain-size increases relative to wavelength, the polarization caused by scattering decreases sharply, especially at infrared wavelengths where Rayleigh scattering due to atoms and molecules becomes negligible. We thus claim that the presence of scattering particles is a necessary but not sufficient condition for observing polarization of emitted light.