Impact of Electron Precipitation on Brown Dwarf Atmospheres and the Missing Auroral H₃⁺ Emission
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1.
California Institute of Technology
Abstract
Recent observations have demonstrated that very low-mass stars and brown dwarfs are capable of sustaining strong magnetic fields despite their cool and neutral atmospheres. These kilogauss field strengths are inferred based on strong, highly circularly polarized gigahertz radio emission, a consequence of the electron cyclotron maser instability. Crucially, these observations imply the existence of energetic nonthermal electron populations, associated with strong current systems, as are found in the auroral regions of the magnetized planets of the solar system. Intense auroral electron precipitation will lead to electron collisions with the H2 gas that should generate the ion H₃⁺. With this motivation, we targeted a sample of ultracool dwarfs, known to exhibit signatures associated with aurorae, in search of the K-band emission features of H₃⁺ using the Keck telescopes on Maunakea. From our sample of nine objects, we found no clear indication of H₃⁺ emission features in our low-to-medium-resolution spectra (R ∼ 3600). We also modeled the impact of an auroral electron beam on a brown dwarf atmosphere, determining the depth at which energetic beams deposit their energy and drive particle impact ionization. We find that the H₃⁺ nondetections can be explained by electron beams of typical energies ≳2–10 keV, which penetrate deeply enough that any H₃⁺ produced is chemically destroyed before radiating energy through its infrared transitions. Strong electron beams could further explain the lack of UV auroral detections and suggest that most or nearly all of the precipitating auroral energy must ultimately emerge as thermal emissions deep in brown dwarf atmospheres.
Copyright and License
© 2024. 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.
Acknowledgement
The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain.
We thank the anonymous reviewer for thoughtful comments in improving this manuscript. J.S.P. would like to thank Chuck Steidel and Gwen Rudie for their assistance preparing observations with MOSFIRE, as well as Nick Konidaris for help with the data reduction. J.S.P. also thanks Michael Eastwood for observing support and Caroline Morley and James Mang for useful model discussions.
J.S.P. was supported by a grant from the National Science Foundation Graduate Research Fellowship under grant No. (DGE-11444469). J.S.P. acknowledges some support for this work under program HST-15924, and JWST-1874 provided by NASA through a grant from the Space Telescope Science Institute, operated by the Association of Universities for Research in Astronomy, Inc., under NSA contract NAS5-26555.
The data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation.
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.
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Additional details
- ISSN
- 1538-4357
- National Science Foundation
- NSF Graduate Research Fellowship DGE-11444469
- National Aeronautics and Space Administration
- HST-15924
- National Aeronautics and Space Administration
- JWST-1874
- National Aeronautics and Space Administration
- NAS5-26555
- W. M. Keck Foundation
- Caltech groups
- Astronomy Department