A detection of Hα in an exoplanetary exosphere

Adam G. Jensen, Seth Redfield, Michael Endl, William D. Cochran, Lars Koesterke, Travis S Barman

Research output: Contribution to journalArticle

45 Citations (Scopus)

Abstract

We report on a search for Hα absorption in four exoplanets. Strong features at Hα are observed in the transmission spectra of both HD 189733b and HD 209458b. We attempt to characterize and remove the effects of stellar variability in HD 189733b, and along with an empirical Monte Carlo test the results imply a statistically significant transit-dependent feature of (- 8.72 ± 1.48) × 10-4 integrated over a 16 Å band relative to the adjacent continuum. We interpret this as the first detection of this line in an exoplanetary atmosphere. A previous detection of Lyα in HD 189733b's atmosphere allows us to calculate an excitation temperature for hydrogen, T exc = 2.6 × 104K. This calculation depends significantly on certain simplifying assumptions. We explore these assumptions and argue that T exc is very likely much greater than the radiative equilibrium temperature (the temperature the planet is assumed to be at based on stellar radiation and the planetary distance) of HD 189733b. A large T exc implies a very low density that is not in thermodynamic equilibrium with the planet's lower atmosphere. We argue that the n = 2 hydrogen required to cause Hα absorption in the atmosphere is created as a result of the greater UV flux at HD189733b, which has the smallest orbit and most chromospherically active central star in our sample. Though the overall integration of HD 209458b's transmission spectrum over a wide band is consistent with zero, it contains a dramatic, statistically significant feature in the transmission spectrum with reflectional symmetry. We discuss possible physical processes that could cause this feature. Our remaining two targets (HD 147506b and HD 149026b) do not show any clear features, so we place upper limits on their Hα absorption levels.

Original languageEnglish (US)
Article number86
JournalAstrophysical Journal
Volume751
Issue number2
DOIs
StatePublished - Jun 1 2012
Externally publishedYes

Fingerprint

exosphere
atmospheres
atmosphere
planets
stellar radiation
lower atmosphere
planet
causes
hydrogen
extrasolar planets
thermodynamic equilibrium
transit
temperature
continuums
broadband
orbits
symmetry
stars
thermodynamics
excitation

Keywords

  • atmospheric effects
  • line: profiles
  • planets and satellites: individual (HD 147506b, HD 149026b, HD 189733b, HD 209458b)
  • stars: individual (HD 147506, HD 149026, HD 189733, HD 209458)
  • techniques: spectroscopic

ASJC Scopus subject areas

  • Space and Planetary Science
  • Astronomy and Astrophysics

Cite this

Jensen, A. G., Redfield, S., Endl, M., Cochran, W. D., Koesterke, L., & Barman, T. S. (2012). A detection of Hα in an exoplanetary exosphere. Astrophysical Journal, 751(2), [86]. https://doi.org/10.1088/0004-637X/751/2/86

A detection of Hα in an exoplanetary exosphere. / Jensen, Adam G.; Redfield, Seth; Endl, Michael; Cochran, William D.; Koesterke, Lars; Barman, Travis S.

In: Astrophysical Journal, Vol. 751, No. 2, 86, 01.06.2012.

Research output: Contribution to journalArticle

Jensen, AG, Redfield, S, Endl, M, Cochran, WD, Koesterke, L & Barman, TS 2012, 'A detection of Hα in an exoplanetary exosphere', Astrophysical Journal, vol. 751, no. 2, 86. https://doi.org/10.1088/0004-637X/751/2/86
Jensen AG, Redfield S, Endl M, Cochran WD, Koesterke L, Barman TS. A detection of Hα in an exoplanetary exosphere. Astrophysical Journal. 2012 Jun 1;751(2). 86. https://doi.org/10.1088/0004-637X/751/2/86
Jensen, Adam G. ; Redfield, Seth ; Endl, Michael ; Cochran, William D. ; Koesterke, Lars ; Barman, Travis S. / A detection of Hα in an exoplanetary exosphere. In: Astrophysical Journal. 2012 ; Vol. 751, No. 2.
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