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arxiv: 1812.02189 · v1 · pith:5E4UXRMFnew · submitted 2018-12-05 · 🌌 astro-ph.EP

Spectrally resolved helium absorption from the extended atmosphere of a warm Neptune-mass exoplanet

R. Allart , V. Bourrier , C. Lovis , D. Ehrenreich , J.J. Spake , A. Wyttenbach , L. Pino , F. Pepe
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D.K. Sing A. Lecavelier des Etangs
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classification 🌌 astro-ph.EP
keywords absorptionheliumatmosphereatmospheresexoplanetextendedneptune-massneutral
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Stellar heating causes atmospheres of close-in exoplanets to expand and escape. These extended atmospheres are difficult to observe because their main spectral signature - neutral hydrogen at ultraviolet wavelengths - is strongly absorbed by interstellar medium. We report the detection of the near-infrared triplet of neutral helium in the transiting warm Neptune-mass exoplanet HAT-P-11b using ground-based, high-resolution observations. The helium feature is repeatable over two independent transits, with an average absorption depth of 1.08+/-0.05%. Interpreting absorption spectra with 3D simulations of the planet's upper atmosphere suggests it extends beyond 5 planetary radii, with a large scale height and a helium mass loss rate =< 3x10^5 g/s. A net blue-shift of the absorption might be explained by high-altitude winds flowing at 3 km/s from day to night-side.

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