Detection of Polarization due to Cloud Bands in the Nearby Luhman 16 Brown Dwarf Binary
Detection of Polarization due to Cloud Bands in the Nearby Luhman 16 Brown Dwarf Binary
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DOI：
10.3847/1538-4357/ab6ef2
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发表时间：
2020-05
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通讯作者：
M. Millar-Blanchaer;J. Girard;T. Karalidi;M. Marley;R. G. van Holstein;S. Sengupta;D. Mawet;T. Kataria;F. Snik;J. D. Boer;R. Jensen-Clem;A. Vigan;S. Hinkley
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作者：
M. Millar-Blanchaer;J. Girard;T. Karalidi;M. Marley;R. G. van Holstein;S. Sengupta;D. Mawet;T. Kataria;F. Snik;J. D. Boer;R. Jensen-Clem;A. Vigan;S. Hinkley
棕矮星呈现出斑片状或空间变化的带状云结构，这是通过光度和光谱变化建模技术推断出来的。然而，这些方法对旋转不变的结构不敏感，例如在木星上看到的带。在这里，我们提出了H波段甚大望远镜/纳科线性偏振测量附近的Luhman 16 L/T过渡二进制，这表明Luhman 16 A具有恒定的纵向云带。该仪器在瞳孔跟踪模式下运行，使我们能够明确区分小的天体物理偏振和0.2%的仪器线性偏振。我们测量了Luhman 16 A和B的线偏振度和角，分别为pA = 0.031% ± 0.004%和λ A = −32° ± 4°，以及pB = 0.010% ± 0.004%和。使用已知的系统的物理参数，我们证明了一个扁圆均匀的大气不能解释在Luhman 16 A中测量的偏振，但可以负责的B分量。通过带状云形态的非穷举搜索，我们证明了一个两条带的情况下，可以实现的线性偏振度为p = 0.03%，并得出结论，测量的偏振的A分量必须主要是由于云带状。对于Luhman 16 B，扁率或云带可能是测量偏振的主要来源。两个双星的偏振角不一致初步表明自旋轨道不一致。这些测量提供了新的证据，云带状褐矮星的流行，而在同一时间展示了一种新的方法，补充光度和光谱变化的方法，用于表征云的形态亚恒星物体没有变化的迹象。
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 , 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.