The effect of Lyman $\alpha$ radiation on mini-Neptune atmospheres around M stars: application to GJ 436b

Abstract

Mini-Neptunes orbiting M stars are a growing population of known exoplanets. Some of them are located very close to their host star, receiving large amounts of UV radiation. Many M stars emit strong chromospheric emission in the H I Lyman $\alpha$ line (Ly$\alpha$) at 1215.67 Å, the brightest far-UV emission line. We show that the effect of incoming Ly$\alpha$ flux can significantly change the photochemistry of mini-Neptunes' atmospheres. We use GJ 436b as an example, considering different metallicities for its atmospheric composition. For solar composition, H${}_2$O-mixing ratios show the largest change because of Ly$\alpha$ radiation. H${}_2$O absorbs most of this radiation, thereby shielding CH${}_4$, whose dissociation is driven mainly by radiation at other far-UV wavelengths ($\sim 1300$ Å). H${}_2$O photolysis also affects other species in the atmosphere, including H, H${}_2$, CO${}_2$, CO, OH and O. For an atmosphere with high metallicity, H${}_2$O- and CO${}_2$-mixing ratios show the biggest change, thereby shielding CH${}_4$. Direct measurements of the UV flux of the host stars are important for understanding the photochemistry in exoplanets' atmospheres. This is crucial, especially in the region between 1 and 10${}^{-6}$ bars, which is the part of the atmosphere that generates most of the observable spectral features.

10 pages, Accepted for publication in MNRAS

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