Limits on the Auroral Generation of H3 + in Brown Dwarf and Extrasolar Giant Planet Atmospheres with the Keck Near Infrared Echelle Spectrograph

The molecular ion H3+ is a potentially powerful tracer of the ionospheres and thermal structures of Jovian planets but has never been detected in a planetary mass object outside of the solar system. Models predict that H3+ emission driven by EUV flux and solar wind on hot Jupiters, or by powerful aurorae on brown dwarfs, will be between 102 and 105 × more intense than that of Jupiter. If optimal conditions for the production of emission do exist, the emission may be detectable by current ground-based instruments or in the near future. We present the first search for H3+ line emission in brown dwarfs with Keck Near Infrared Echelle Spectrograph L′ high-resolution spectroscopy. Additionally, we survey stars hosting giant planets at semimajor axes near 0.1–0.2 au, which models suggest may be the best planetary targets. No candidate H3+ emission is found. The limits we place on the emission of H3+ from brown dwarfs indicate that auroral generation of H3+ in these environments likely does not linearly scale from the processes found on Jupiter, plausibly due to deeper atmospheric penetration by precipitating auroral electrons. Detection of H3+ emission in brown dwarfs may be possible with the James Webb Space Telescope, or future 33 m class telescopes.