Detection of Polarization due to Cloud Bands in the Nearby Luhman 16 Brown Dwarf Binary

Abstract

Brown dwarfs exhibit patchy or spatially varying banded cloud structures that are inferred through photometric and spectroscopic variability modeling techniques. However, these methods are insensitive to rotationally invariant structures, such as the bands seen in Jupiter. Here, we present H-band Very Large Telescope/NaCo linear polarization measurements of the nearby Luhman 16 L/T transition binary, which suggest that Luhman 16A exhibits constant longitudinal cloud bands. The instrument was operated in pupil tracking mode, allowing us to unambiguously distinguish between a small astrophysical polarization and the ∼2% instrumental linear polarization. We measure the degree and angle of linear polarization of Luhman 16A and B to be p A = 0.031% ± 0.004% and ψ A = −32° ± 4°, and p B = 0.010% ± 0.004% and , respectively. Using known physical parameters of the system, we demonstrate that an oblate homogeneous atmosphere cannot account for the polarization measured in Luhman 16A, but could be responsible for that of the B component. Through a nonexhaustive search of banded cloud morphologies, we demonstrate a two-banded scenario that can achieve a degree of linear polarization of p = 0.03% and conclude that the measured polarization of the A component must be predominantly due to cloud banding. For Luhman 16B, either oblateness or cloud banding could be the dominant source of the measured polarization. The misaligned polarization angles of the two binary components tentatively suggest spin–orbit misalignment. These measurements provide new evidence for the prevalence of cloud banding in brown dwarfs while at the same time demonstrating a new method—complementary to photometric and spectroscopic variability methods—for characterizing the cloud morphologies of substellar objects without signs of variability.

Atmospheric circulation of brown dwarfs and directly imaged exoplanets driven by cloud radiative feedback: global and equatorial dynamics

“…We find systematic equator-to-pole differences of clouds and temperature structures due to the latitudinal variation of the Coriolis parameter , supporting recent observations by Vos et al (2017) and Vos et al (2020). Large-scale equatorial disturbances may help to explain the nature of longitudinal variation in Doppler mapping (Crossfield et al 2014) and time-varying polarization (Millar-Blanchaer et al 2020) of Luhman 16B.…”

“…Due to the latitudinal variation of the cloud thickness at a given rotation period and the dependence of global-average cloud thickness on varying rotation period, the different viewing angles and the variation of rotation periods of the field BDs might contribute to the scatter of observed near-IR colors. Millar-Blanchaer et al (2020) showed that assuming two broad zonal bands with different cloud properties in each hemisphere, they can reproduce time-averaged polarization measured for the nearby BD Luhman 16B. Our models do not show clear zonally banded cloud structure like those in Jupiter and Saturn, but exhibit smooth equator-to-pole cloud thickness variations.…”

Aerosols in Exoplanet Atmospheres

“…Polarization of the thermal emission of brown dwarfs and directly imaged exoplanets can also constrain aerosol properties and distributions in their atmospheres (e.g. Sengupta & Krishan, 2001;Marley & Sengupta, 2011;Stolker et al, 2017;Sanghavi & Shporer, 2018;Millar-Blanchaer et al, 2020).…”

Atmospheric circulation of brown dwarfs and directly imaged exoplanets driven by cloud radiative feedback: effects of rotation

“…Fourth, large-scale circulation provides a source of vertical mixing that helps to explain the inferred chemical disequilibrium in a wide range of BDs (e.g., Saumon et al 2006;Stephens et al 2009;Leggett et al 2016aLeggett et al , 2019Miles et al 2020), especially in stratified atmospheres where convection does not play a direct role in mixing. Other techniques detecting or constraining the presence of global circulation of BDs include Doppler imaging (Crossfield et al 2014), simultaneous tracking of near-IR and radio variability (Allers et al 2020) and precise near-IR polarization measurements (Millar-Blanchaer et al 2020).…”