2021MNRAS.507.3626K

The signs of an expanding atmosphere of HD209458b have been observed with far-ultraviolet transmission spectroscopy and in the measurements of transit absorption by metastable He I. These observations are interpreted using the hydrodynamic and Monte Carlo numerical simulations of various degree of complexity and consistency. At the same time, no attempt has been made to model atmospheric escape of a magnetized HD209458b, to see how the planetary magnetic field might affect the measured transit absorption lines. This paper presents the global 3D MHD self-consistent simulations of the expanding upper atmosphere of HD209458b interacting with the stellar wind, and models the observed H I (Ly α), O I (1306 Å), C II (1337 Å), and He I (10 830 Å) transit absorption features. We find that the planetary dipole magnetic field with the equatorial surface value of B_p = 1 G profoundly changes the character of atmospheric material outflow and the related absorption. We also investigate the formation of planetary magnetosphere in the stellar wind and show that its size is more determined by the escaping atmosphere flow rather than by the strength of magnetic field. Fitting of the simulation results to observations enables constraining the stellar XUV flux and He abundance at F_XUV ∼ 10 erg cm² s^–1 (at 1 au) and He/H ≃ 0.02, respectively, as well as setting an upper limit for the dipole magnetic field of B_p < 0.1 G on the planetary surface at the equator. This implies that the magnetic dipole moment u_P of HD209458b should be less than 6 per cent of the Jovian value.