Modeling Polarization Signals from Cloudy Brown Dwarfs: Luhman 16 A and B in Three Dimensions

TL;DR

This study uses 3D atmospheric models and radiative transfer calculations to accurately reproduce polarization signals from Luhman 16A and B, revealing complex cloud structures and variability over time.

Contribution

It demonstrates the effectiveness of 3D GCMs combined with radiative transfer in modeling brown dwarf polarization, highlighting the importance of complex cloud vortices.

Findings

Polarization signals match observations with specific cloud particle sizes.

Polarization varies on hour-long timescales due to rotation.

Simple band models over-predict polarization without vortices.

Abstract

The detection of disk-integrated polarization from Luhman 16A and B in H-band, and subsequent modeling, has been interpreted in the framework of zonal cloud bands on these bodies. Recently, Tan and Showman (2021) investigated three-dimensional atmospheric circulation and cloud structures of brown dwarfs with general circulation models (GCMs), and their simulations yield complex cloud distributions showing some aspects of zonal jets, but also complex vortices that cannot be captured by a simple model. Here we use these 3D GCMs specific to Luhman 16A and B, along with the three-dimensional Monte Carlo radiative transfer code ARTES, to calculate their polarization signals. We adopt the 3D temperature-pressure and cloud profiles from the GCMs as our input atmospheric structures. Our polarization calculations at 1.6 $\mu$m agree well with the measured degree of linear polarization from both Luhman 16 A and B. Our calculations reproduce the measured polarization for both the objects with cloud particle sizes between 0.5-1 \,$\mu$m for Luhman 16 A and 5 \,$\mu$m for Luhman 16 B. We find that the degree of linear polarization can vary on hour-long timescales over the course of a rotation period. We also show that models with azimuthally symmetric band-like cloud geometries, typically used for interpreting polarimetry observations of brown dwarfs, over-predict the polarization signal if the cloud patterns do not include complex vortices within these bands. This exploratory work shows that GCMs are promising for modeling and interpreting polarization signals of brown dwarfs.