Abstract
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.
Highlights
Brown dwarfs occupy a unique parameter space, with effective temperatures (Teff), masses, and radii in between those of giant exoplanets and stars
Both the near-exact repetition of a light curve feature seen in two Hubble Space Telescope (HST) data sets separated by over a year (Karalidi et al 2016), as well as the change in state of Luhman 16A from low variability to high variability (Buenzli et al 2015a) could both be explained with variable cloud bands with slightly different periods, beating over time
We have presented new H-band linear polarization measurements of the Luhman 16 binary brown dwarf system obtained with Very Large Telescope (VLT)/NaCo
Summary
Brown dwarfs occupy a unique parameter space, with effective temperatures (Teff), masses, and radii in between those of giant exoplanets and stars. Brown dwarfs at the L/T spectral-type transition are believed to undergo an evolution from extremely dusty/cloudy atmospheres, where the clouds are mostly made of corundum, iron, and silicates, to nearly clear atmospheres that eventually begin to form clouds from other families of condensates such as Cr, MnS, Na2S, ZnS, and KCl (Burgasser et al 2002; Marley et al 2010; Morley et al 2012).
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