Simulation of 10 830 Å absorption with a 3D hydrodynamic model reveals the solar He abundance in upper atmosphere of WASP-107b

ABSTRACT 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.