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 H-3 (+) 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 H-3 (+) emission driven by EUV flux and solar wind on hot Jupiters, or by powerful aurorae on brown dwarfs, will be between 10(2) and 10(5) x 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 H-3 (+) 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 H-3 (+) emission is found. The limits we place on the emission of H-3 (+) from brown dwarfs indicate that auroral generation of H-3 (+) 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 H-3 (+) emission in brown dwarfs may be possible with the James Webb Space Telescope, or future 33 m class telescopes.

Automated summary

H-3(+) molecular ions are potentially powerful tracers of ionospheres and thermal structures in gas giant planets, but they have not been detected in planetary mass objects outside the solar system. The emission of H-3(+) on hot Jupiters and brown dwarfs is predicted to be much more intense than that of Jupiter. In this study, we searched for H-3(+) emission in brown dwarfs using high-resolution spectroscopy and surveyed stars hosting giant planets. However, no candidate emission was found.