Impact of Electron Precipitation on Brown Dwarf Atmospheres and the Missing Auroral Emission

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 H _2 gas that should generate the ion ${{\rm{H}}}_{3}^{+}$ . 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 ${{\rm{H}}}_{3}^{+}$ using the Keck telescopes on Maunakea. From our sample of nine objects, we found no clear indication of ${{\rm{H}}}_{3}^{+}$ 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 ${{\rm{H}}}_{3}^{+}$ nondetections can be explained by electron beams of typical energies ≳2–10 keV, which penetrate deeply enough that any ${{\rm{H}}}_{3}^{+}$ 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.