2021MNRAS.503L..23K

Transmission spectroscopy of WASP-107b revealed 7-8 per cent absorption at the position of metastable He I triplet at 10 830 Å in the velocity range of [-20; 10] km s^–1. This absorption is stronger than that measured in other exoplanets. With a dedicated 3D self-consistent hydrodynamic multifluid model, we simulated the expanding upper atmosphere of WASP-107b and reproduced within the observations accuracy the measured He I absorption profiles, constraining the stellar X-ray and UV radiation (XUV) flux to 6-10 erg cm^–2 s^–1 at 1 au, and the upper atmosphere helium abundance He/H to 0.075-0.15. The radiation pressure acting on the metastable He I atoms was shown to be an important factor influencing the absorption profiles. Its effect is counterbalanced by the collisional depopulation of the He I metastable state. Altogether, the observed He I absorption in WASP-107b can be interpreted with the expected reasonable parameters of the stellar-planetary system and appropriate account of the electron and atom impact processes.