Abstract
Abstract 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 self-consistently 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.
Highlights
The study of planets beyond the Solar system, or exoplanets, is one of the fast-growing and intriguing fields in the space science
In spite of the fact that the radiation pressure is in principle able to accelerate the escaping planetary hydrogen atoms up to the velocities typical for the observed transit spectral features, our hydrodynamic model shows that under the conditions of HD209458b, the amount of such accelerated particles is not sufficient to produce the measurable effects in the Lyα absorption line at the velocities of the order of –100 km/s
In the present paper we simulated the Lyα absorption line at Doppler shifted velocities of the order of 100 km/s for the transits of HD209458b using for the first time the self-consistent 2D hydrodynamic model of the expanding PW which interacts with the plasma flow of SW in the vicinity of the planet
Summary
The study of planets beyond the Solar system, or exoplanets, is one of the fast-growing and intriguing fields in the space science. As proposed by Holmström et al (2008) and Ekenbäck et al (2010), the escaping planetary atoms are ionized in the flow of stellar wind (further SW) protons via charge-exchange reaction This mechanism produces fast hydrogen atoms (i.e., ENAs), necessary to explain the asymmetric absorption in the blue spectral wing of Lyα, not by acceleration of particles, but due to the presence in the SW of fast and hot protons. We found that for the used parameters of the model, such as the Solar type XUV flux of 4.466 erg cm-2 s-1 at 1 a.u. and the typical for the Sun density, velocity and temperature of the SW resulting in a pressure of about 5 10 6 bar the amount of ENAs, obtained in simulations of HD209458b, appears an order of magnitude smaller than needed to influence the transit observations in Lyα line. In our simulations we apply the whole equation (2) to calculate Lyα line absorption in order to account for self-shielding of radiation pressure
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