Publication Date:
2014-11-13
Description:
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 α line (Lyα) at 1215.67 Å, the brightest far-UV emission line. We show that the effect of incoming Lyα 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 Lyα 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 (~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.
Print ISSN:
0035-8711
Electronic ISSN:
1365-2966
Topics:
Physics
Permalink