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    A Detection Of H Alpha In An Exoplanetary Exosphere

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    2012_06_exoplanetaryexosphere.pdf (1.425Mb)
    Date
    2012-06
    Author
    Jensen, Adam G.
    Redfield, Seth
    Endl, Michael
    Cochran, William D.
    Koesterke, Lars
    Barman, Travis
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    Abstract
    We report on a search for H alpha absorption in four exoplanets. Strong features at H alpha 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) x 10(-4) integrated over a 16 angstrom band relative to the adjacent continuum. We interpret this as the first detection of this line in an exoplanetary atmosphere. A previous detection of Ly alpha in HD 189733b's atmosphere allows us to calculate an excitation temperature for hydrogen, T-exc = 2.6 x 10(4) K. 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 H alpha absorption in the atmosphere is created as a result of the greater UV flux at HD 189733b, 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 H alpha absorption levels.
    Department
    Astronomy
    Subject
    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
    spin-orbit alignment
    transiting extrasolar planet
    bright k-star
    mu-m
    transmission spectrum
    emission-spectrum
    dayside spectrum
    stellar
    activity
    hot jupiters
    lyman-alpha
    astronomy & astrophysics
    URI
    http://hdl.handle.net/2152/35106
    Citation
    Jensen, Adam G., Seth Redfield, Michael Endl, William D. Cochran, Lars Koesterke, and Travis Barman. "A Detection of H? in an Exoplanetary Exosphere." The Astrophysical Journal, Vol. 751, No. 2 (Jun., 2012): 86.
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