Signatures of X-ray dominated chemistry in the spectra of exoplanetary atmospheres

Daniele Locci; Giambattista Aresu; Antonino Petralia; Giuseppina Micela; Antonio Maggio; Cesare Cecchi-Pestellini

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arXiv · 2402.04688

Signatures of X-ray dominated chemistry in the spectra of exoplanetary atmospheres

Daniele Locci·Giambattista Aresu·Antonino Petralia·Giuseppina Micela·Antonio Maggio·Cesare Cecchi-Pestellini

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Abstract

High-energy radiation from stars impacts planetary atmospheres deeply affecting their chemistry, providing departures from chemical equilibrium. While the upper atmospheric layers are dominated by ionizations induced by extreme ultraviolet radiation, deeper into the atmosphere molecular abundances are controlled by a characteristic X-ray dominated chemistry, mainly driven by an energetic secondary electron cascade. In this work, we aim at identifying molecular photochemically induced fingerprints in the transmission spectra of a giant planet atmosphere. We have developed a numerical code capable of synthesizing transmission spectra with arbitrary spectral resolution, exploiting updated infrared photoabsorption cross sections. Chemical mixing ratios are computed using a photochemical model, tailored to investigate high energy ionization processes. We find that in case of high levels of stellar activity, synthetic spectra in both low and high resolutions show significant, potentially observable out-of-equilibrium signatures arising mainly from CO, CH$_4$, C$_2$H$_2$, and HCN.

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Daniele Locci, Giambattista Aresu, Antonino Petralia, Giuseppina Micela, Antonio Maggio, Cesare Cecchi-Pestellini. 2024-02-07. Signatures of X-ray dominated chemistry in the spectra of exoplanetary atmospheres. https://arxiv.org/abs/2402.04688

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