A CRIRES-search for H₃⁺emission from the hot Jupiter atmosphere of HD 209458 b

Abstract

Close-in extrasolar giant planets are expected to cool their thermospheres by producing H + 3 emission in the near-infrared (NIR), but simulations predict H + 3 emission intensities that differ in the resulting intensity by several orders of magnitude. We want to test the observability of H + 3 emission with CRIRES at the Very Large Telescope (VLT), providing adequate spectral resolution for planetary atmospheric lines in NIR spectra. We search for signatures of planetary H + 3 emission in the L band, using spectra of HD 209458 obtained during and after secondary eclipse of its transiting planet HD 209458 b. We searched for H + 3 emission signatures in spectra containing the combined light of the star and, possibly, the planet. With the information on the ephemeris of the transiting planet, we derive the radial velocities at the time of observation and search for the emission at the expected line positions. We also apply a cross-correlation test to search for planetary signals and use a shift and add technique combining all observed spectra taken after secondary eclipse to calculate an upper emission limit. We do not find signatures of atmospheric H + 3 emission in the spectra containing the combined light of HD 209458 and its orbiting planet. We calculate the emission limit for the H + 3 line at 3953.0 nm [Q(1, 0)] to be 8.32 × 10 18 W and a limit of 5.34 × 10 18 W for the line at 3985.5 nm [Q(3, 0)]. Comparing our emission limits to the theoretical predictions suggests that we lack 1 to 3 magnitudes of sensitivity to measure H + 3 emission in our target object. We show that under more favorable weather conditions the data quality can be improved significantly, reaching 5 × 10 16 W for star-planet systems that are close to Earth. We estimate that pushing the detection limit down to 10 15 W will be possible with ground-based observations with future instrumentation, for example, the European Extremly Large Telescope.

Ions in the Thermosphere of Exoplanets: Observable Constraints Revealed by Innovative Laboratory Experiments

“…The abundance and distribution of + H 3 , the stable ionic form of H 2 , is essential to understand the chemistry and get information about the thermal structure, dynamic, and energy balance of exoplanet atmospheres (Miller et al 2000). The + H 3 has been thoroughly studied in the mid-IR on Jupiter (Stallard et al 2001), Saturn (Geballe et al 1993), and Uranus (Trafton et al 1993), but detection in the thermosphere of hot Jupiter exoplanets, with a predicted mixing ratio ranging from 10 −6 to 10 −4 , so far remains illusive (Shkolnik et al 2006;Lenz et al 2016). Helling & Rimmer (2019) assumed that the formation pathway of + H 3 through the ionization of H 2 (R11), although very efficient, is difficult in hot exoplanet atmospheres due to the thermal decomposition of H 2 , which becomes important when the temperature reaches 1000 K (Yelle 2004).…”

Lightning and charge processes in brown dwarf and exoplanet atmospheres

“…Such localized interaction of an ionospheric environment with a local magnetic field has been traced through H + 3 emission on Jupiter [52]. Lenz et al [37] reported a non-detection of H +…”

“…Predictions of significant H + 3 concentrations in Jupiter's upper atmosphere [30,31] led to observational programmes that successfully detected H + 3 in Jupiter in situ [32] and remotely [33]. Likewise, the ionization of exoplanet atmospheres is predicted to lead to the formation of upper atmospheric H + 3 [34], and this prediction has led to several observational programmes to search for H + 3 in Hot Jupiter atmospheres, thus far without success [35][36][37]. Although there are spectral hints of H + 3 in brown dwarf atmospheres [38], no definitive detection of H + 3 has been made in a brown dwarf atmosphere, and there have been as of yet no published results from a programme In what follows, we discuss environmental processes that affect the ionization of exoplanet and brown dwarf atmospheres that cause the formation of a global or partial ionosphere ( §2).…”

Hydrogen molecular ions: H 3 + , H 5 + and beyond

“…The claimed serendipitous observation of H + 3 in the cooling remnants of supernova SN1987a [21] is supported by chemical models [22] but, of course, cannot be repeated. However, despite its perceived importance for cooling and stabilizing hot Jupiter exoplanets [23], there remains no definitive observation of H + 3 in either an exoplanet [24][25][26] or a brown dwarf [27]. The H + 3 molecular ion is the simplest stable molecular ion of hydrogen.…”

“…The original discovery of H + 3 occurred over a century ago [26] but, as this issue demonstrates, there clearly remain a whole host of fundamental issues and their implications to be studied, both using and involving the molecular ions of hydrogen. In the introduction to the 1989 paper announcing the detection of H + 3 in the atmosphere of Jupiter, Drossart and his co-workers predicted: 'Such strong H + 3 lines could be used in future ground-based monitoring of the Jovian auroral activity and to search for this molecule in the interstellar medium' ( [1], p. 539).…”