Modeling the photo-polarimetric characteristics of brown dwarfs
Abstract
An envelope of scatterers like free electrons, atoms/molecules, or haze/clouds affect the Stokes vector of radiation emitted by an oblate body.Due to their high rotation rates, brown dwarfs (BDs) are often considerably oblate. We present a conics-based radiative transfer (RT) scheme for computing the disc-resolved and disc-integrated polarized emission of an oblate body like a BD or extrasolar giant planet (EGP) bearing homogenous or patchy clouds. Using this capability, we theoretically examine the photo-polarimetric signal of BDs as a function of the scattering properties of its atmosphere like cloud optical thickness and grain size concurrently with BD properties like oblateness and inclination angle. The effect of oblateness is examined with and without the temperature gradients caused by gravitational darkening, revealing that the latter can considerably amplify the disc-integrated polarization. The signal depends on both oblateness and inclination angle, with the degree of polarization (DoP) increasing with oblateness and decreasing with inclination, a property useful for assessing the exact spatial orientation of the rotation axis in favorable cases. Our examination of BD cloud properties shows a relative blue-shift in the near-infrared (NIR) for increasing droplet size in optically thick clouds - interesting in view of the observed relative brightening in the J-band for L/T transition BDs. For large cloud grains, the polarization decreases sharply, while the transmitted intensity shows a steady increase, thus reducing the DoP.
- Publication:
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American Astronomical Society Meeting Abstracts #231
- Pub Date:
- January 2018
- Bibcode:
- 2018AAS...23133401S