To shed more light on the nature of the observed Lyα absorption during transits of HD 209458b and to quantify the major mechanisms responsible for the production of fast hydrogen atoms (the so-called energetic neutral atoms, ENAs) around the planet, 2D hydrodynamic multifluid modeling of the expanding planetary upper atmosphere, which is driven by stellar XUV, and its interaction with the stellar wind has been performed. The model selfconsistently describes the escaping planetary wind, taking into account the generation of ENAs due to particle acceleration by the radiation pressure and by the charge exchange between the stellar wind protons and planetary atoms. The calculations in a wide range of stellar wind parameters and XUV flux values showed that under typical Sun-like star conditions, the amount of generated ENAs is too small, and the observed absorption at the level of 6%-8% can be attributed only to the non-resonant natural line broadening. For lower XUV fluxes, e.g., during the activity minima, the number of planetary atoms that survive photoionization and give rise to ENAs increases, resulting in up to 10%-15% absorption at the blue wing of the Lyα line, caused by resonant thermal line broadening. A similar asymmetric absorption can be seen under the conditions realized during coronal mass ejections, when sufficiently high stellar wind pressure confines the escaping planetary material within a kind of bowshock around the planet. It was found that the radiation pressure in all considered cases has a negligible contribution to the production of ENAs and the corresponding absorption.

Using a global 3D, fully self-consistent, multi-fluid hydrodynamic model, we simulate the escaping upper atmosphere of the warm Neptune GJ436b, driven by the stellar XUV radiation impact and gravitational forces and interacting with the stellar wind. Under the typical parameters of XUV flux and stellar wind plasma expected for GJ436, we calculate in-transit absorption in Lyα and find that it is produced mostly by Energetic Neutral Atoms outside of the planetary Roche lobe, due to the resonant thermal line broadening. At the same time, the influence of radiation pressure has been shown to be insignificant. The modelled absorption is in good agreement with the observations and reveals such features as strong asymmetry between blue and red wings of the absorbed Lyα line profile, deep transit depth in the high velocity blue part of the line reaching more than 70%, and the timing of early ingress. On the other hand, the model produces significantly deeper and longer egress than in observations, indicating that there might be other processes and factors, still not accounted, that affect the interaction between the planetary escaping material and the stellar wind. At the same time, it is possible that the observational data, collected in different measurement campaigns, are affected by strong variations of the stellar wind parameters between the visits, and therefore, they cannot be reproduced altogether with the single set of model parameters.Keywords: hydrodynamicsplasmasplanets and satellites: individual: exoplanetsplanets and satellites: physical evolutionplanets and satellites: atmosphereplanet-star interactions 2004, García Muñoz 2007, Koskinen et al. 2007) clarified the basic physics of the escaping upper atmosphere in the form of planetary wind (further PW), which includes the XUV heating, hydrogen plasma photo-chemistry, radiation cooling, gravitational and thermal pressure forces. They helped to explain some of the in-transit spectral observations by the presence of an expanded partially ionized upper atmospheres, which fill the Roche lobes of hot giant exoplanets, such as HD209458b and HD189733b (Ben-Jaffel 2007, Ben-Jaffel & Sona Hosseini 2010, Koskinen et al. 2007. These expanding atmospheres were shown to be sufficiently dense to produce the absorption in Lyα due to natural line broadening mechanism.However, the detection of absorption in the resonant lines of heavy elements such as OI, CII, and SiIII , Linsky et al. 2010, has shown that the absorbing material of planetary origin far beyond the Roche lobe has to be considered as well (Ben-Jaffel & Sona Hosseini 2010, Shaikhislamov et al. 2018a. The presence of a huge hydrogen corona also is a prerequisite for the explanation of strong in-transit Lyα absorption of GJ436b. By this, there is another crucial factor, besides of the Roche lobe effect, which has to be properly taken into account in the modeling of large-scale plasma dynamics around the close-orbit exoplanetsthe stellar wind (further SW) plasma. Self-consistent description of the escaping multi-co...

We present a 3D fully self-consistent multi-fluid hydrodynamic aeronomy model to study the structure of a hydrogen dominated expanding upper atmosphere around the hot Jupiter HD 209458b and the warm Neptune GJ 436b. In comparison to previous studies with 1D and 2D models, the present work finds such 3D features as zonal flows in upper atmosphere reaching up to 1 km/s, the tilting of the planetary outflow by Coriolis force by up to 45 degrees and its compression around equatorial plane by tidal forces. We also investigated in details the influence of Helium (He) on the structure of the thermosphere. It is found that by decrease of the barometric scale-height, the He presence in the atmosphere strongly affects the H 2 dissociation front and the temperature maximum.

The absorption of stellar radiation observed by HD 209458b in the resonant lines of O i and C ii has not yet been satisfactorily explained. We apply a 2D hydrodynamic multi-fluid model that self-consistently describes the expanding planetary wind, driven by stellar XUV radiation and influenced by tidal forces and the surrounding stellar wind. According to this model, HD 209458b has a hydrogen-dominated plasmasphere, expanding beyond the Roche lobe, in the form of two supersonic streams that propagate toward and away from the star. The species heavier than hydrogen and helium are dragged in the escaping material streams and accelerated up to 50 km s−1. Our simulations show that, assuming solar abundances, O i and C ii produce absorption due to the Doppler resonance mechanism at the level of 6%–10%, which is consistent with the observations. Most of this absorption takes place in the streams. The transit depth in the O i and C ii lines is unaffected by the stellar wind, unless it is strong enough to form a compact bowshock around the planet and able to redirect all the escaping material to the tail. In this case, the absorption profile becomes asymmetric due to the prominent blueshifted attenuation. Thus, the spectroscopic measurements enable probing of the planetary wind character, as well as the strength of the stellar wind. The computed absorption at wavelengths of the Si iii, Mg i, and Mg ii lines at solar abundances appears to be much stronger, compared to the observations. This possibly indicates that Si and Mg may be under-abundant in the upper atmosphere of HD 209458b.

Context. Because of its proximity to an active K-type star, the hot Jupiter WASP-80b has been identified as a possible excellent target for detecting and measuring He I absorption in the upper atmosphere. Aims. Our aim was to look for, and eventually measure and model, metastable He I atmospheric absorption. Methods. We observed four primary transits of WASP-80b in the optical and near-infrared using the HARPS-N and GIANO-B high-resolution spectrographs attached to the Telescopio Nazionale Galileo telescope, focusing the analysis on the He I triplet. We further employed a three-dimensional hydrodynamic aeronomy model to understand the observational results. Results. We did not find any signature of planetary absorption at the position of the He I triplet with an upper limit of 0.7% (i.e. 1.11 planetary radii; 95% confidence level). We re-estimated the high-energy stellar emission, which we combined with a stellar photospheric model, to generate the input for the hydrodynamic modelling. We determined that, assuming a solar He to H abundance ratio, He I absorption should have been detected. Considering a stellar wind 25 times weaker than solar, we could reproduce the non-detection only by assuming a He to H abundance ratio about 16 times smaller than solar. Instead, considering a stellar wind ten times stronger than solar, we could reproduce the non-detection only with a He to H abundance ratio about ten times smaller than solar. We attempted to understand this result by collecting all past He I measurements and looking for correlations with high-energy stellar emission and planetary gravity, but without success. Conclusions. WASP-80b is not the only planet with an estimated sub-solar He to H abundance ratio, which suggests the presence of efficient physical mechanisms (e.g. phase separation, magnetic fields) capable of significantly modifying the He to H content in the upper atmosphere of hot Jupiters. The planetary macroscopic properties and the shape of the stellar spectral energy distribution are not sufficient for predicting the presence or absence of detectable metastable He in a planetary atmosphere, since the He abundance also appears to play a major role.