dc.description.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 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 pA = 0.031% ± 0.004% and
ψA = −32° ± 4°, and pB = 0.010% ± 0.004% and y b=
73 +13 -11
13 , 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. |
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