A Detection Of H-alpha In An Exoplanetary Exosphere
Adam G. Jensen, Seth Redfield, Michael Endl, William D. Cochran, Lars, Koesterke, and Travis S. Barman

TL;DR
This study reports the first detection of H-alpha absorption in an exoplanetary atmosphere, specifically in HD 189733b, indicating excited hydrogen possibly caused by stellar UV flux, with implications for atmospheric composition and excitation conditions.
Contribution
First detection of H-alpha absorption in an exoplanet atmosphere, providing insights into hydrogen excitation and atmospheric conditions influenced by stellar activity.
Findings
H-alpha absorption detected in HD 189733b's atmosphere
Excitation temperature of hydrogen estimated at 2.6x10^4 K
No significant H-alpha features detected in HD 209458b, HD 147506b, and HD 149026b
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)x10^-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=2.6x10^4 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…
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