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Published August 2018 | Submitted + Published
Journal Article Open

The Strongest Magnetic Fields on the Coolest Brown Dwarfs


We have used NSF's Karl G. Jansky Very Large Array to observe a sample of five known radio-emitting late-L and T dwarfs ranging in age from ~0.2 to 3.4 Gyr. We observed each target for seven hours, extending to higher frequencies than previously attempted and establishing proportionally higher limits on maximum surface magnetic field strengths. Detections of circularly polarized pulses at 8–12 GHz yield measurements of 3.2–4.1 kG localized magnetic fields on four of our targets, including the archetypal cloud variable and likely planetary-mass object T2.5 dwarf SIMP J01365663+0933473. We additionally detect a pulse at 15–16.5 GHz for the T6.5 dwarf 2MASS 10475385+2124234, corresponding to a localized 5.6 kG field strength. For the same object, we tentatively detect a 16.5–18 GHz pulse, corresponding to a localized 6.2 kG field strength. We measure rotation periods between ~1.47–2.28 hr for 2MASS J10430758+2225236, 2MASS J12373919+6526148, and SDSS J04234858–0414035, supporting (i) an emerging consensus that rapid rotation may be important for producing strong dipole fields in convective dynamos, and/or (ii) rapid rotation is a key ingredient for driving the current systems powering auroral radio emission. We observe evidence of variable structure in the frequency-dependent time series of our targets on timescales shorter than a rotation period, suggesting a higher degree of variability in the current systems near the surfaces of brown dwarfs. Finally, we find that age, mass, and temperature together cannot account for the strong magnetic fields produced by our targets.

Additional Information

© 2018 The American Astronomical Society. Received 2017 August 2; revised 2018 May 2; accepted 2018 May 3; published 2018 July 31. M.M.K. thanks Jackie Villadsen for helping to troubleshoot calibrations and Rakesh Yadav for thoughtful and instructive discussions about dynamo modeling. M.M.K. additionally thanks the enthusiastically supportive staff at the National Radio Astronomy Observatory for their technical mentorship. Support for this work was provided by the NSF through the Grote Reber Fellowship Program administered by Associated Universities, Inc./National Radio Astronomy Observatory. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. This material is based in part upon work supported by the National Science Foundation under Grant AST-1654815 and the NASA Solar System Exploration Virtual Institute cooperative agreement 80ARC017M0006. G.H. acknowledges the support of the Alfred P. Sloan Foundation and the Research Corporation for Science Advancement. J.S.P. was supported by a grant from the National Science Foundation Graduate Research Fellowship under grant no. DGE-1144469. This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation. Facility: VLA - Very Large Array. Software: CASA (McMullin et al. 2007), MATLAB (MATLAB Signal Processing Toolbox, R2016a).

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Published - Kao_2018_ApJS_237_25.pdf

Submitted - 1808.02485.pdf


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August 19, 2023
October 18, 2023