Signatures of Nitrogen Chemistry in Hot Jupiter Atmospheres

Ryan J. MacDonald, Nikku Madhusudhan
(Submitted on 1 Nov 2017)

Inferences of molecular compositions of exoplanetary atmospheres have generally focused on C, H, and O-bearing molecules. Recently, additional absorption in HST WFC3 transmission spectra around 1.55μm has been attributed to nitrogen-bearing chemical species: NH3 or HCN. These species, if present in significant abundance, would be strong indicators of disequilibrium chemical processes — e.g. vertical mixing and photochemistry. The derived N abundance, in turn, could also place important constraints on planetary formation mechanisms. Here, we examine the detectability of nitrogen chemistry in exoplanetary atmospheres. In addition to the WFC3 bandpass (1.1-1.7μm), we find that observations in K-band at ∼2.2μm, achievable with present ground-based telescopes, sample a strong NH3 feature, whilst observations at ∼3.1μm and ∼4.0μm sample strong HCN features. In anticipation of such observations, we predict absorption feature amplitudes due to nitrogen chemistry in the 1-5μm spectral range possible for a typical hot Jupiter. Finally, we conduct atmospheric retrievals of 9 hot Jupiter transmission spectra in search of near-infrared absorption features suggestive of nitrogen chemistry. We report weak detections of NH3 in WASP-31b (2.2σ), HCN in WASP-63b (2.3σ), and excess absorption that could be attributed to NH3 in HD 209458b. High-precision observations from 1-5μm (e.g., with the James Webb Space Telescope), will enable definitive detections of nitrogen chemistry, in turn serving as powerful diagnostics of disequilibrium atmospheric chemistry and planetary formation processes.

Comments:    9 pages, 5 figures. Accepted for publication in ApJL
Subjects:    Earth and Planetary Astrophysics (astro-ph.EP)
Cite as:    arXiv:1711.00467 [astro-ph.EP] (or arXiv:1711.00467v1 [astro-ph.EP] for this version)
Submission history
From: Ryan MacDonald
[v1] Wed, 1 Nov 2017 18:00:00 GMT (16429kb,D)
https://arxiv.org/abs/1711.00467
Astrobiology
Astrochemistry