The Astrophysical Journal (Jan 2023)
Nitrogen as a Tracer of Giant Planet Formation. II. Comprehensive Study of Nitrogen Photochemistry and Implications for Observing NH3 and HCN in Transmission and Emission Spectra
Abstract
Atmospheric nitrogen may provide important constraints on giant planet formation. Following our semianalytical work, we further pursue the relation between observable NH _3 and an atmosphere’s bulk nitrogen abundance by applying the photochemical kinetics model VULCAN across planetary equilibrium temperature, mass, age, eddy diffusion coefficient, atmospheric composition, and stellar spectral type. We confirm that the quenched NH _3 abundance coincides with the bulk nitrogen abundance only at sub-Jupiter-mass (≲1 M _J ) planets and old ages (≳1 Gyr) for solar composition atmospheres, highlighting important caveats for inferring atmospheric nitrogen abundances. Our semianalytical model reproduces the quenched NH _3 abundance computed by VULCAN and thus helps to infer the bulk nitrogen abundance from a retrieved NH _3 abundance. By computing transmission and emission spectra, we predict that the equilibrium temperature range of 400–1000 K is optimal for detecting NH _3 because NH _3 depletion by thermochemistry and photochemistry is significant at hotter planets whereas entire spectral features become weak at colder planets. For Jupiter-mass planets around Sun-like stars in this temperature range, NH _3 leaves observable signatures of ∼50 ppm at 1.5, 2.1, and 11 μ m in transmission spectra and >300–100 ppm at 6 and 11 μ m in emission spectra. The photodissociation of NH _3 leads HCN to replace NH _3 at low pressures. However, the low HCN column densities lead to much weaker absorption features than for NH _3 . The NH _3 features are readily accessible to JWST observations to constrain atmospheric nitrogen abundances, which may open a new avenue to understanding the formation processes of giant exoplanets.
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