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

TL;DR

This study investigates how Lyman alpha radiation from M stars influences the photochemistry of mini-Neptune atmospheres, using GJ 436b as a case, revealing significant effects on water and methane chemistry depending on metallicity.

Contribution

It demonstrates the impact of stellar Lyman alpha radiation on exoplanet atmospheric chemistry, emphasizing the importance of UV flux measurements for accurate atmospheric modeling.

Findings

Lyman alpha radiation significantly alters water and methane abundances.

Water absorbs Lyα radiation, shielding methane from dissociation.

High metallicity atmospheres show larger compositional changes.

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 \AA, 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 ($\sim1300$ \AA). 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.