Abstract
Abstract The interaction of exoplanets with their host stars causes a vast diversity in bulk and atmospheric compositions and physical and chemical conditions. Stellar radiation, especially at the shorter wavelengths, drives the chemistry in the upper atmospheric layers of close orbiting gaseous giants, providing drastic departures from equilibrium. In this study, we aim at unfolding the effects caused by photons in different spectral bands on the atmospheric chemistry. This task is particularly difficult because the characteristics of chemical evolution emerge from many feedbacks on a wide range of timescales, and because of the existing correlations among different portions of the stellar spectrum. In describing the chemistry, we have placed particular emphasis on the molecular synthesis induced by X-rays. The weak X-ray photoabsorption cross sections of the atmospheric constituents boost the gas ionization to pressures inaccessible to vacuum and extreme-ultraviolet photons. Although X-rays interact preferentially with metals, they produce a secondary electron cascade able to ionize efficiently hydrogen- and helium-bearing species, giving rise to a distinctive chemistry.
Highlights
Exoplanets form and evolve under the influence of their host stars
We aim at unfolding the effects caused by photons in different spectral bands on the atmospheric chemistry, with particular emphasis on the molecular synthesis induced by X-rays
We adopt: (1) a hot plasma thermal emission for photon energies comprised between 1 keV and 100 eV (X-rays), with the integrated luminosity, equation (6), and the plasma temperature, TX being free parameters; (2) a constant spectrum in the Extreme UltraViolet (EUV) band 100 < E < 13.6 eV, whose luminosity scales directly with the X-ray luminosity according to the Sanz-Forcada et al (2011) relation, LEUV = 6.31 × 104 L0X.86; (3) a Lyman−α profile at 10.2 eV, whose intensity is related to the X-ray luminosity by the expression LLyα = 4.57 × 1016 L0X.43 derived from data reported in Linsky et al (2020); (4) a PHOENIX G-type star model providing the flux in the UV spectral range between 3 and 13.6 eV
Summary
Exoplanets form and evolve under the influence of their host stars. In the process, planetary atmospheres naturally arise and modify under selective environmental constraints, providing an astonishing diversity in compositions, and physical and chemical conditions. Not many works include detailed descriptions of the secondary electron cascade (e.g., Cecchi-Pestellini et al 2006; Shematovich et al 2014) Additional ionizing sources, such as cosmic rays and stellar energetic particles are addressed in Airapetian et al (2016, 2017) and Barth et al (2021), while lightning and charge processes in Helling & Rimmer (2019). The effects produced by secondary electrons are by far more important than the corresponding ionization, excitation, and dissociation events caused directly by X-rays (e.g., Locci et al 2018). This is a consequence of the large primary photoelectron energies. In the last Section we discuss the results and outline our conclusions
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