High-energy radiation from stars impacts planetary atmospheres, deeply affecting their chemistry and 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 the 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, CH₄, C₂H₂, and HCN.

中文翻译：

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系外行星大气光谱中 X 射线主导的化学特征

来自恒星的高能辐射会影响行星大气层，深刻影响其化学成分并偏离化学平衡。虽然上层大气层主要由极紫外辐射引起的电离作用主导，但在大气层深处，分子丰度由特征性的 X 射线主导化学物质控制，主要由高能二次电子级联驱动。在这项工作中，我们的目标是识别巨行星大气透射光谱中分子光化学诱导的指纹。我们开发了一种数字代码，能够利用更新的红外光吸收截面来合成具有任意光谱分辨率的透射光谱。使用专为研究高能电离过程而定制的光化学模型来计算化学混合比。我们发现，在恒星活动水平较高的情况下，低分辨率和高分辨率的合成光谱均显示出显着的、可能可观察到的不平衡特征，主要来自 CO、CH ₄、C ₂ H ₂和 HCN。