Issue |
A&A
Volume 527, March 2011
|
|
---|---|---|
Article Number | A110 | |
Number of page(s) | 10 | |
Section | Planets and planetary systems | |
DOI | https://doi.org/10.1051/0004-6361/201015698 | |
Published online | 04 February 2011 |
The upper atmosphere of the exoplanet HD 209458 b revealed by the sodium D lines
Temperature-pressure profile, ionization layer, and thermosphere
1
CNRS, UPMC, Institut d’astrophysique de Paris, UMR 7095,
98bis boulevard Arago,
75014
Paris,
France
e-mail: alfred@iap.fr
2
Astrophysics Group, School of Physics, University of Exeter,
Stocker Road,
Exeter
EX4 4QL,
UK
3
Harvard–Smithsonian Center for Astrophysics, 60 Garden St.,
Cambridge,
MA
02138,
USA
4
Laboratoire d’Astrophysique de Grenoble, Université Joseph
Fourier, CNRS (UMR 5571), BP
53, 38041
Grenoble Cedex 9,
France
5
Laboratoire d’Astrophysique de Marseille,
Technopôle Marseille Étoile, 38 rue
Frédéric Joliot Curie, 13013
Marseille,
France
Received:
6
September
2010
Accepted:
18
November
2010
A complete reassessment of the Hubble Space Telescope (HST)
observations of the transits of the extrasolar planet HD 209458 b has provided a
transmission spectrum of the atmosphere over a wide range of wavelengths. Analysis of the
NaI absorption line profile has already shown that the sodium abundance has to drop by at
least a factor of ten above a critical altitude. Here we analyze the profile in the deep
core of the NaI doublet line from HST and high-resolution ground-based spectra to further
constrain the vertical structure of the HD 209458 b atmosphere. With a
wavelength-dependent cross section that spans more than 5 orders of magnitude, we use the
absorption signature of the NaI doublet as an atmospheric probe. The NaI transmission
features are shown to sample the atmosphere of HD 209458 b over an altitude range of more
than 6500 km, corresponding to a pressure range of 14 scale heights spanning 1 millibar to
10-9 bar pressures. By comparing the observations with a multi-layer model in
which temperature is a free parameter at the resolution of the atmospheric scale height,
we constrain the temperature vertical profile and variations in the Na abundance in the
upper part of the atmosphere of HD 209458 b. We find a rise in temperature above the drop
in sodium abundance at the 3 mbar level. We also identify an isothermal atmospheric layer
at 1500 ± 100 K spanning almost 6 scale heights in altitude, from 10-5 to
10-7 bar. Above this layer, the temperature rises again to
K at ~10-9 bar, indicating the presence of a thermosphere. The resulting
temperature-pressure (T-P) profile agrees with the Na condensation scenario at the 3 mbar
level, with a possible signature of sodium ionization at higher altitudes, near the
3 × 10-5 bar level. Our T-P profile is found to be in good agreement with the
profiles obtained with aeronomical models including hydrodynamic escape.
Key words: stars: individual: HD 209458 / planets and satellites: atmospheres / techniques: spectroscopic / methods: observational / methods: data analysis
© ESO, 2011
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