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

Abstract

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.

Patchy Forsterite Clouds in the Atmospheres of Two Highly Variable Exoplanet Analogs

“…In Jupiter and Saturn, the collisions between electrons and the atmospheric gases lead to optical and UV emissions due to excitation and ionization processes, and the subsequent ion chemistry leads to the formation of strongly emitting + H 3 (Perry et al 1999;Vasavada et al 1999) ions. It is likely that the auroral activity in the atmospheres of brown dwarfs also leads to the formation of detectable amounts of + H 3 , but this has not yet been observed (Pineda et al 2017;Gibbs & Fitzgerald 2022). In this work, we can speculate on whether the auroral activity on SIMP J0136+09 could lead to the suppression of H 2 O and CO absorption.…”

“…These simulations show that, for the case of LSR−1835, small amounts of + H 3 ions can form at pressures down to ∼0.1 bar, but that + H 3 reacts rapidly with H 2 O and CO, and can produce the relatively stable ion hydronium, H 3 O + , at pressures down to ∼1 bar. Gibbs & Fitzgerald (2022) recently provided upper (bottom). The predicted spectral variability driven by our retrieved patchy forsterite cloud slab is shown by the blue line for SIMP J0136+09 and green line for 2MASS J2139+02.…”

ExoMol line lists – L: high-resolution line lists of H3+, H2D+, D2H+, and D3+

“…The infrared spectrum of H + 3 has been extensively observed in giant planets in our solar system such as Jupiter (Drossart et al 1989;Ballester et al 1994;Miller et al 1997;Moore et al 2017), Saturn (Geballe et al 1993;Stallard et al 2008a,b), Uranus (Trafton et al 1993;Lam et al 1997;Trafton et al 1999;Melin et al 2019) and is believed to be ⋆ The corresponding author: j.tennyson@ucl.ac.uk present in Neptune (Melin et al 2011(Melin et al , 2018 although it is yet to be detected there. Its presence can be used as an effective temperature probe here and in other astrophysical settings (Gibbs & Fitzgerald 2022). H + 3 is similarly expected to be of importance in extrasolar giant planets (Chadney et al 2016;Khodachenko et al 2015), such as hot-Jupiters (Lenz et al 2016), and an even more prominent feature in the aurorae of brown dwarfs (Gibbs & Fitzgerald 2022); however, it has so far defied observation on these objects.…”

“…Its presence can be used as an effective temperature probe here and in other astrophysical settings (Gibbs & Fitzgerald 2022). H + 3 is similarly expected to be of importance in extrasolar giant planets (Chadney et al 2016;Khodachenko et al 2015), such as hot-Jupiters (Lenz et al 2016), and an even more prominent feature in the aurorae of brown dwarfs (Gibbs & Fitzgerald 2022); however, it has so far defied observation on these objects.…”

Evidence for a radiation belt around a brown dwarf

“…The bursting component has been linked to the electron cyclotron maser instability [ECMI ( 2 )] mechanism, a coherent radio emission process that is responsible for auroral radio emission on the planets of the Solar System ( 3 , 4 ). However, the details of the emission mechanism may vary between UCDs and planets ( 5 , 6 ). The quiescent component is usually interpreted as being caused by radio emission from mildly or ultrarelativistic electrons spiraling in magnetic fields [gyrosynchrotron and synchrotron mechanisms, respectively ( 7 , 8 )], which could originate in the UCD’s corona and/or radiation belts ( 9 – 11 ).…”