A Detection Of H-alpha In An Exoplanetary Exosphere

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 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 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.