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Article Dans Une Revue Astronomy and Astrophysics - A&A Année : 2020

The high-energy environment and atmospheric escape of the mini-Neptune K2-18 b

Résumé

K2-18 b is a transiting mini-Neptune that orbits a nearby (38 pc), cool M3 dwarf and is located inside its region of temperate irradiation. We report on the search for hydrogen escape from the atmosphere K2-18 b using Lyman-α transit spectroscopy with the Space Telescope Imaging Spectrograph instrument installed on the Hubble Space Telescope. We analyzed the time-series of fluxes of the stellar Lyman-α emission of K2-18 in both its blue-and redshifted wings. We found that the average blueshifted emission of K2-18 decreases by 67% ± 18% during the transit of the planet compared to the pre-transit emission, tentatively indicating the presence of H atoms escaping vigorously and being blown away by radiation pressure. This interpretation is not definitive because it relies on one partial transit. Based on the reconstructed Lyman-α emission of K2-18, we estimate an EUV irradiation in the range 10 1 − 10 2 erg s −1 cm −2 and a total escape rate on the order of 10 8 g s −1. The inferred escape rate suggests that the planet will lose only a small fraction (< 1%) of its mass and retain its volatile-rich atmosphere during its lifetime. More observations are needed to rule out stellar variability effects, confirm the in-transit absorption, and better assess the atmospheric escape and high-energy environment of K2-18 b.
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Dates et versions

hal-03104121 , version 1 (08-01-2021)

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Leonardo A dos Santos, David Ehrenreich, Vincent Bourrier, Nicola Astudillo-Defru, Xavier Bonfils, et al.. The high-energy environment and atmospheric escape of the mini-Neptune K2-18 b. Astronomy and Astrophysics - A&A, 2020, 634, pp.L4. ⟨10.1051/0004-6361/201937327⟩. ⟨hal-03104121⟩
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