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We simulate with a global 3D aeronomy code two warm Neptunes in the TOI-421 system and show that both planets experience significant escape of their upper atmospheres. The double shock structures, generated around the planets in course of their interaction with the stellar wind (SW) plasma flow are revealed. The calculations of stellar Ly α transit absorption by the planets reveal that it reaches a detectable level only for a moderate or strong SW, with a sufficiently high density. In this case, the energetic neutral atoms provide significant absorption at the high velocity blue wing of the Ly α line, whereas the corresponding transit light curves exhibit an early ingress and extended egress features. With the same code, we also modelled the absorption at the position of the 10 830 Å line of the metastable helium, showing that it can be detected only for the farthest planet of the considered two, if the helium abundance is comparable to the solar value. [ABSTRACT FROM AUTHOR]

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 Bp = 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 Bp < 0.1 G on the planetary surface at the equator. This implies that the magnetic dipole moment µP of HD209458b should be less than 6 per cent of the Jovian value. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The absorption of stellar radiation observed at transits of HD 209458b in resonant lines of O  i and C  ii has not yet been satisfactorily modelled. In our previous two-dimensional (2D) simulations, we have shown that the hydrogen-dominated upper atmosphere of HD 209458b, heated by extreme ultraviolet radiation, expands supersonically beyond the Roche lobe and drags heavier species along with it. Assuming solar abundances, O  i and C  ii particles accelerated by tidal forces to velocities up to 50  |${\rm km}\,{\rm s}^{-1}$| should produce the absorption due to Doppler resonance at the level of 6–10 per  cent, consistent with the observations. Since the 2D geometry does not take into account the Coriolis force in a planet reference frame, the question remains to what extent the spiralling of escaping planetary material and actually achieved velocity may influence the conclusions made on the basis of 2D modelling. In this paper, we apply for the first time in studies of HD 209458b a global 3D hydrodynamic multifluid model. The results confirm our previous findings that the velocity of the planetary flow is sufficiently high to match the widths of O  i and C  ii resonant lines. To match the absorption in those lines, including H  i , with observations, the mass-loss rate of HD 209458b should be about |$2.5\times10^{11}\,{\rm g\,s^{-1}}$|⁠ , which is 3.5 times larger than that revealed by the 2D model. The other novel finding is that matching of the absorption measured in Mg  ii and S  iii lines requires at least 10 times lower abundances of these elements than the solar values. [ABSTRACT FROM AUTHOR]