Modelling the He I triplet absorption at 10 830 Å in the atmosphere of HD 209458 b
HD 209458 b is an exoplanet with an upper atmosphere undergoing blow-off escape that has mainly been studied using measurements of the Ly<i>α<i/> absorption. Recently, high-resolution measurements of absorption in the He I triplet line at 10 830 Å of several exoplanets (including HD 209...
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| Main Authors: | , , , , , , , , , , , , , , , , , , |
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| Format: | Article (Journal) |
| Language: | English |
| Published: |
07 April 2020
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| In: |
Astronomy and astrophysics
Year: 2020, Volume: 636 |
| ISSN: | 1432-0746 |
| DOI: | 10.1051/0004-6361/201937175 |
| Online Access: | Verlag, lizenzpflichtig, Volltext: https://doi.org/10.1051/0004-6361/201937175 Verlag, lizenzpflichtig, Volltext: https://www.aanda.org/articles/aa/abs/2020/04/aa37175-19/aa37175-19.html 
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| Author Notes: | M. Lampón, M. López-Puertas, L.M. Lara, A. Sánchez-López, M. Salz, S. Czesla, J. Sanz-Forcada, K. Molaverdikhani, F.J. Alonso-Floriano, L. Nortmann, J.A. Caballero, F.F. Bauer, E. Pallé, D. Montes, A. Quirrenbach, E. Nagel, I. Ribas, A. Reiners, and P.J. Amado |
| Summary: | HD 209458 b is an exoplanet with an upper atmosphere undergoing blow-off escape that has mainly been studied using measurements of the Ly<i>α<i/> absorption. Recently, high-resolution measurements of absorption in the He I triplet line at 10 830 Å of several exoplanets (including HD 209458 b) have been reported, creating a new opportunity to probe escaping atmospheres.<i>Aims.<i/> We aim to better understand the atmospheric regions of HD 209458 b from where the escape originates.<i>Methods.<i/> We developed a 1D hydrodynamic model with spherical symmetry for the HD 209458 b thermosphere coupled with a non-local thermodynamic model for the population of the He I triplet state. In addition, we performed high-resolution radiative transfer calculations of synthetic spectra for the helium triplet lines and compared them with the measured absorption spectrum in order to retrieve information about the atmospheric parameters.<i>Results.<i/> We find that the measured spectrum constrains the [H]/[H<sup>+<sup/>] transition altitude occurring in the range of 1.2 <i>R<i/><sub>P<sub/>-1.9 <i>R<i/><sub>P<sub/>. Hydrogen is almost fully ionised at altitudes above 2.9 <i>R<i/><sub>P<sub/>. We also find that the X-ray and extreme ultraviolet absorption takes place at effective radii from 1.16 to 1.30 <i>R<i/><sub>P<sub/>, and that the He I triplet peak density occurs at altitudes from 1.04 to 1.60 <i>R<i/><sub>P<sub/>. Additionally, the averaged mean molecular weight is confined to the 0.61-0.73 g mole<sup>−1<sup/> interval, and the thermospheric H/He ratio should be larger than 90/10, and most likely approximately 98/2. We also provide a one-to-one relationship between mass-loss rate and temperature. Based on the energy-limited escape approach and assuming heating efficiencies of 0.1-0.2, we find a mass-loss rate in the range of (0.42-1.00) ×10<sup>11<sup/> g s<sup>−1<sup/> and a corresponding temperature range of 7125-8125 K.<i>Conclusions.<i/> The analysis of the measured He I triplet absorption spectrum significantly constrains the thermospheric structure of HD 209458 b and advances our knowledge of its escaping atmosphere. |
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| Item Description: | Gesehen am 20.10.2020 |
| Physical Description: | Online Resource |
| ISSN: | 1432-0746 |
| DOI: | 10.1051/0004-6361/201937175 |