On the Existence of Energetic Atoms in the Upper Atmosphere of Exoplanet HD209458b

Abstract

Stellar irradiation and particle forcing strongly affect the immediate environment of extrasolar giant planets orbiting near their parent stars. However, it is not clear how the energy is deposited over the planetary atmosphere, nor how the momentum and energy spaces of the different species that populate the system are modified. Here, we use far-ultraviolet emission spectra from HD209458 in the wavelength range (1180-1710) Å to bring new insight to the composition and energetic processes in play in the gas nebula around the transiting planetary companion. In that frame, we consider up-to-date atmospheric models of the giant exoplanet where we implement non-thermal line broadening to simulate the impact on the transit absorption of superthermal atoms (H I, O I, and C II) populating the upper layers of the nebula. Our sensitivity study shows that for all existing models, a significant line broadening is required for O I and probably for C II lines in order to fit the observed transit absorptions. In that frame, we show that O I and C II are preferentially heated compared to the background gas with effective temperatures as large as T _{O I} /T_B ~ 10 for O I and T _{C II} /T_B ~ 5 for C II. By contrast, the situation is much less clear for H I because several models could fit the Lyα observations including either thermal H I in an atmosphere that has a dayside vertical column [H I] ~ 1.05 × 10²¹ cm^-2, or a less extended thermal atmosphere but with hot H I atoms populating the upper layers of the nebula. If the energetic H I atoms are either of stellar origin or populations lost from the planet and energized in the outer layers of the nebula, our finding is that most models should converge toward one hot population that has an H I vertical column in the range [H I]_hot ~ (2-4) × 10¹³ cm^-2 and an effective temperature in the range T _{H I} ~ (1-1.3) × 10⁶ K, but with a bulk velocity that should be rather slow.