Detection of polarization due to cloud bands in the nearby Luhman 16 brown dwarf binary

Brown dwarfs exhibit patchy or spatially-varying banded cloud structures, that are inferred through photometric and spectroscopic variability modeling techniques. However, these methods are insensi- tive to rotationally-invariant structures, such as the bands seen in Jupiter. Here, we present H-band VLT/NaCo linear polarization measurements of the nearby Luhman 16 L/T transition binary, that suggest that Luhman 16A exhibits constant longitudinal cloud bands. The instrument was oper- ated 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 p = 0.010 ± 0.004% and ψ = 73+13◦, respectively. Using known physical parameters of the system, B B−11 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 non-exhaustive 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 suggests 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.褐矮星（Brown dwarfs）具有斑块状或空间分布的带状云结构，此类结构可通过测光与光谱变分建模技术反推得到。然而，这类方法对旋转不变结构并不敏感，例如木星上观测到的云带。本文针对邻近的卢曼16（Luhman 16）L/T过渡双星开展了H波段甚大望远镜/NAOS-CONICA（VLT/NaCo）线性偏振测量，结果显示卢曼16A存在稳定的纵向云带。观测仪器采用光斑跟踪模式，使得我们能够明确区分微弱的天体物理偏振与约2%的仪器线性偏振。我们测得卢曼16A与B的线性偏振度及偏振角分别为：$p_A = 0.031 pm 0.004\%$，$psi_A = -32 pm 4^circ$；$p_B = 0.010 pm 0.004\%$，$psi_B = 73^circ_{-11}^{+13}$。利用该双星系统已知的物理参数，我们证明扁球形均匀大气无法解释卢曼16A测得的偏振信号，却可用于解释B星的偏振结果。通过对带状云形态的非穷尽搜索，我们发现双带模型可实现$p=0.03\%$的线性偏振度，由此推断A组分测得的偏振信号主要源于云带结构。对于卢曼16B而言，扁球形效应或云带结构均可作为其偏振信号的主要来源。两个双星组分的偏振角存在显著偏移，初步暗示存在自旋-轨道错位现象。本研究为褐矮星中云带结构的普遍性提供了新的观测证据，同时也提出了一种新方法——作为测光与光谱变分方法的补充——用于表征无明显变分迹象的亚恒星天体的云形态。