1 research outputs found
On the existence of energetic atoms in the upper atmosphere of exoplanet HD209458b
Stellar irradiation and particles forcing strongly affect the immediate
environment of extrasolar giant planets orbiting near their parent stars. Here,
we use far-ultraviolet emission spectra from HD209458 in the wavelength range
(1180-1710)A 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 CII) 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~10 for OI and T_{CII}/T_B~5 for CII. By
contrast, the situation is much less clear for HI because several models could
fit the Lyman-a observations including either thermal HI in an atmosphere that
has a dayside vertical column [HI]~1.05x10^{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}(2-4)x10^{13} cm^{-2} and an
effective temperature in the range T_{HI}(1-1.3)x10^6 K, but with a bulk
velocity that should be rather slow.Comment: 15 pages, 10 figures, corrected for typos, references remove