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

Millar-Blanchaer, Maxwell A. and Girard, Julien H. and Karalidi, Theodora and Marley, Mark S. and van Holstein, Rob G. and Sengupta, Sujan and Mawet, Dimitri and Kataria, Tiffany and Snik, Frans and de Boer, Jos and Jensen-Clem, Rebecca and Vigan, Arthur and Hinkley, Sasha (2020) Detection of Polarization due to Cloud Bands in the Nearby Luhman 16 Brown Dwarf Binary. Astrophysical Journal, 894 (1). Art. No. 42. ISSN 0004-637X.

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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 ψ/B = 73⁺¹³₋₁₁°, 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.

Item Type:Article
Related URLs:
URLURL TypeDescription
Millar-Blanchaer, Maxwell A.0000-0001-6205-9233
Girard, Julien H.0000-0001-8627-0404
Karalidi, Theodora0000-0001-7356-6652
Marley, Mark S.0000-0002-5251-2943
Sengupta, Sujan0000-0002-6176-3816
Mawet, Dimitri0000-0002-8895-4735
Kataria, Tiffany0000-0003-3759-9080
de Boer, Jos0000-0002-1028-7522
Jensen-Clem, Rebecca0000-0003-0054-2953
Vigan, Arthur0000-0002-5902-7828
Hinkley, Sasha0000-0001-8074-2562
Additional Information:© 2020 The Author(s). Published by the American Astronomical Society. Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Received 2019 May 21; revised 2020 January 8; accepted 2020 January 13; published 2020 May 5. We thank D. Saumon for updated brown dwarf evolutionary models. Support for this work was provided by NASA through the NASA Hubble Fellowship grant HST-HF2-51378.001-A awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS5-26555. The research of F.S. and J.d.B. leading to these results has received funding from the European Research Council under ERC Starting Grant agreement 678194 (FALCONER). Based on observations collected at the European Southern Observatory under ESO programmes 0101.C-0561(A) and 0101.C-0561(B). (Part of) The research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. This work is supported by the National Science Foundation under grant No. AAG-1816341. Facility: VLT(NaCo). - Software: astropy (Astropy Collaboration et al. 2013), photutils (Bradley et al. 2017), emcee (Foreman-Mackey et al. 2013), NumPy, SciPy, Matplotlib.
Group:Astronomy Department
Funding AgencyGrant Number
NASA Hubble FellowshipHST-HF2-51378.001-A
European Research Council (ERC)678194
Subject Keywords:Near infrared astronomy ; Very Large Telescope ; Polarimetry
Issue or Number:1
Classification Code:Unified Astronomy Thesaurus concepts: Near infrared astronomy (1093); Very Large Telescope (1767); Polarimetry (1278)
Record Number:CaltechAUTHORS:20200501-084127580
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Official Citation:Maxwell A. Millar-Blanchaer et al 2020 ApJ 894 42
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:102957
Deposited By: George Porter
Deposited On:05 May 2020 15:50
Last Modified:05 May 2020 15:50

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