  Transmission spectroscopy of WASP-107b revealed 7-8% absorption at the
position of metastable HeI triplet at 10830 {\AA} in Doppler velocity range of
[-20; 10] km/s, which is stronger than that measured in other exoplanets. With
a dedicated 3D self-consistent hydrodynamic multi-fluid model we calculated the
expanding upper atmosphere of WASP-107b and reproduced within the observations
accuracy the measured HeI absorption profiles, constraining the stellar XUV
flux to (6-10) erg cm-2 s-1 at 1 a.u., and the upper atmosphere helium
abundance He/H to 0.075-0.15. The radiation pressure acting on the metastable
HeI atoms was shown to be an important factor affecting the shape of the
absorption profiles. Its effect is counterbalanced by the processes of
collisional depopulation of the HeI metastable state. Altogether, the observed
HeI 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.
