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The numerical simulation of the HD 63433 system is performed with the aim to study upper atmospheres of two mini-Neptunes, planets b and c, interacting with the stellar wind of the parent star. The obtained results demonstrate that both exoplanets form the extended envelopes with strong supersonic outflows. The synthetic absorption profiles in the Ly α line show that under moderate stellar wind conditions, similar to those of the normal solar wind, the energetic neutral atoms contribute to the absorption in the high-velocity blue wing of the line at a level of tens per cent. The absorption in metastable helium He  i (23S) line appears rather weak and below the detection limit by current instruments. An important feature revealed by the simulations is that the tail of escaping atmospheric material of the inner planet disturbs the stellar wind at orbital location of the outer planet and might, therefore, affect its observation in Ly α line. [ABSTRACT FROM AUTHOR]

Possible reasons for the non-detection of absorption in the metastable He  i (23S) line at transit observations of warm Neptune GJ436b, in spite of the well-pronounced strong absorption features measured earlier in Lyα for this planet, are investigated. We perform numeric simulations of the escaping upper atmosphere of this planet and its He  i (23S) triplet absorption with a global 3D multifluid, self-consistent hydrodynamic model. By fitting the model parameters to the lowest detection level of absorption measurements, we constrain an upper limit for the He/H abundance three times smaller than the solar value. We demonstrate that neither the significant changes of the stellar wind related to possible stellar coronal mass ejections (CMEs), or possible variations in the stellar ionization radiation, nor the presence of heavy trace elements have a crucial effect on the absorption at the 10   830 Å line of He  i (23S) triplet. The main reason of weak signature is that the region populated by the absorbing metastable helium is rather small (<3 R p), as well as the small size of the planet itself in comparison to the host star. We show that the radiation pressure force acting on the He  i (23S) atoms spreads them along the line of sight and around the planet, thus further reducing peak absorption. [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]