ON THE EXISTENCE OF ENERGETIC ATOMS IN THE UPPER ATMOSPHERE OF EXOPLANET HD209458b

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) angstrom 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 (HI, OI, 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 OI and probably for CII lines in order to fit the observed transit absorptions. In that frame, we show that OI and CII are preferentially heated compared to the background gas with effective temperatures as large as T-OI/T-B similar to 10 for OI and T-CII/T-B similar to 5 for CII. By contrast, the situation is much less clear for HI because several models could fit the Ly alpha observations including either thermal HI in an atmosphere that has a dayside vertical column [HI] similar to 1.05 x 10(21) cm(-2), or a less extended thermal atmosphere but with hot HI atoms populating the upper layers of the nebula. If the energetic HI 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 HI vertical column in the range [HI](hot) similar to (2-4) x 10(13) cm(-2) and an effective temperature in the range T-HI similar to (1-1.3) x 10(6) K, but with a bulk velocity that should be rather slow.