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Energetic neutral atoms as the explanation for the high-velocity hydrogen around HD 209458b
Institutet för rymdfysik (IRF).
2008 (English)In: Nature, ISSN 0028-0836, Vol. 451, 970-972 p.Article in journal (Refereed) Published
##### Abstract [en]

Absorption in the stellar Lyman- $\alpha$(Ly$\alpha$) line observed during the transit of the extrasolar planet HD 209458b in front of its host star reveals high-velocity atomic hydrogen at great distances from the planet1, 2. This has been interpreted as hydrogen atoms escaping from the planet's exosphere1, 3, possibly undergoing hydrodynamic blow-off4, and being accelerated by stellar radiation pressure. Energetic neutral atoms around Solar System planets have been observed to form from charge exchange between solar wind protons and neutral hydrogen from the planetary exospheres5, 6, 7, however, and this process also should occur around extrasolar planets. Here we show that the measured transit-associated Ly$\alpha$ absorption can be explained by the interaction between the exosphere of HD 209458b and the stellar wind, and that radiation pressure alone cannot explain the observations. As the stellar wind protons are the source of the observed energetic neutral atoms, this provides a way of probing stellar wind conditions, and our model suggests a slow and hot stellar wind near HD 209458b at the time of the observations. line observed during the transit of the extrasolar planet HD 209458b in front of its host star reveals high-velocity atomic hydrogen at great distances from the planet1, 2. This has been interpreted as hydrogen atoms escaping from the planet's exosphere1, 3, possibly undergoing hydrodynamic blow-off4, and being accelerated by stellar radiation pressure. Energetic neutral atoms around Solar System planets have been observed to form from charge exchange between solar wind protons and neutral hydrogen from the planetary exospheres5, 6, 7, however, and this process also should occur around extrasolar planets. Here we show that the measured transit-associated Ly$\alpha$ absorption can be explained by the interaction between the exosphere of HD 209458b and the stellar wind, and that radiation pressure alone cannot explain the observations. As the stellar wind protons are the source of the observed energetic neutral atoms, this provides a way of probing stellar wind conditions, and our model suggests a slow and hot stellar wind near HD 209458b at the time of the observations.

##### Place, publisher, year, edition, pages
2008. Vol. 451, 970-972 p.
##### Identifiers
OAI: oai:DiVA.org:umu-3363DiVA: diva2:142030
Available from: 2008-09-04 Created: 2008-09-04 Last updated: 2009-06-22Bibliographically approved
##### In thesis
1. Numerical modelling of ENAs from stellar wind interactions
Open this publication in new window or tab >>Numerical modelling of ENAs from stellar wind interactions
2008 (English)Doctoral thesis, comprehensive summary (Other academic)
##### Abstract [en]

Energetic neutral atoms (ENAs) are produced whenever a stellar wind encounters a neutral atmosphere. If a stellar wind proton comes sufficiently close to a neutral a charge-exchange reaction may take place, transforming the proton into an ENA. Unaffected by magnetic and electric fields, ENAs provide an opportunity for global imaging of stellar wind interactions.

This thesis presents methods and results of numerical modelling of stellar wind interactions. In particular it treats in depth production of ENAs at comets, Mars and the extrasolar planet HD 209458b.

Sufficiently accurate numerical models of stellar wind interactions require extensive computations. Parallel computing has therefore been used throughout the work, both for fluid and particle simulations of space plasmas. This thesis describes the use of a general simulation tool, providing parallel computing for space plasma simulations.

The thesis presents estimations of the magnitude and morphology of the ENA production at comets and HD 209458b. It compares the results obtained with observations and analyzes them in the light of ENA production at similar objects. Also, simulated ENA images for Mars were produced and compared to observations.

##### Place, publisher, year, edition, pages
Kiruna: Institutet för rymdfysik (IRF), 2008. 52 p.
##### Series
IRF Scientific Report, ISSN 0284-1703 ; 297
##### Keyword
Space physics, Space technology, Astrophysics
##### National Category
Aerospace Engineering
##### Identifiers
urn:nbn:se:umu:diva-1804 (URN)978-91-977255-5-2 (ISBN)
##### Public defence
2008-09-26, Aulan, Institutet för rymdfysik, Etian 100, Kiruna, 10:00
##### Supervisors
Available from: 2008-09-04 Created: 2008-09-03Bibliographically approved

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#### Other links

Publisher's full texthttp://dx.doi.org/10.1038/nature06600
Nature

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