Volume 634, February 2020
|Number of page(s)||5|
|Section||Letters to the Editor|
|Published online||31 January 2020|
Letter to the Editor
The high-energy environment and atmospheric escape of the mini-Neptune K2-18 b⋆
Observatoire Astronomique de l’Université de Genève, 51 Chemin des Maillettes, 1290 Versoix, Switzerland
2 Departamento de Matemática y Física Aplicadas, Universidad Católica de la Santísima Concepción, Alonso de Rivera, 2850 Concepción, Chile
3 Université Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
4 Laboratoire de Météorologie Dynamique, Institut Pierre Simon Laplace, Université Paris 6 Boite Postale 99, 75252 Paris Cedex 05, France
Accepted: 13 January 2020
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 101 − 102 erg s−1 cm−2 and a total escape rate on the order of 108 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.
Key words: stars: individual: K2-18 / stars: chromospheres / planets and satellites: atmospheres / ISM: kinematics and dynamics
The HST Lyman-α spectra are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (126.96.36.199) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/634/L4
© ESO 2020
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