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On the origin of the non-detection of metastable HeI in the upper atmosphere of the hot Jupiter WASP-80b

L. FossatiI. PillitteriI. F. Shaikhislamov ...+4 M. S. Rumenskikh
Mar 2023
We aim to narrow down the origin of the non-detection of the metastable HeItriplet at about 10830 A obtained for the hot Jupiter WASP-80b. We measure theX-ray flux of WASP-80 from archival observations and use it as input to scalingrelations accounting for the coronal [Fe/O] abundance ratio to infer theextreme-ultraviolet (EUV) flux in the 200-504 A range, which controls theformation of metastable HeI. We run three dimensional (magneto) hydrodynamicsimulations of the expanding planetary upper atmosphere interacting with thestellar wind to study the impact on the HeI absorption of the stellarhigh-energy emission, the He/H abundance ratio, the stellar wind, and thepossible presence of a planetary magnetic field up to 1 G. For a low stellarEUV emission, which is favoured by the measured logR'HK value, the HeInon-detection can be explained by a solar He/H abundance ratio in combinationwith a strong stellar wind, or by a sub-solar He/H abundance ratio, or by acombination of the two. For a high stellar EUV emission, the non-detectionimplies a sub-solar He/H abundance ratio. A planetary magnetic field isunlikely to be the cause of the non-detection. The low EUV stellar flux, drivenby the low [Fe/O] coronal abundance, is the likely primary cause of the HeInon-detection. High-quality EUV spectra of nearby stars are urgently needed toimprove the accuracy of high-energy emission estimates, which would then enableone to employ the observations to constrain the planetary He/H abundance ratioand the stellar wind strength. This would greatly enhance the information thatcan be extracted from HeI atmospheric characterisation observations.