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标题：The high-energy environment and atmospheric escape of the mini-Neptune K2-18 b ⋆
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作者：Leonardo A. dos Santos ; David Ehrenreich ; Vincent Bourrier 等
期刊名称：Astronomy & Astrophysics
印刷版ISSN：0004-6361
电子版ISSN：1432-0746
出版年度：2020
卷号：634
页码：1-5
DOI：10.1051/0004-6361/201937327
语种：English
出版社：EDP Sciences
摘要：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-αtransit spectroscopy with the Space Telescope Imaging Spectrograph instrument installed on theHubbleSpace Telescope. We analyzed the time-series of fluxes of the stellar Lyman-αemission 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-αemission of K2-18, we estimate an EUV irradiation in the range 101 − 102erg s−1cm−2and a total escape rate on the order of 108g 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.
关键词：enstars: individual: K2-18stars: chromospheresplanets and satellites: atmospheresISM: kinematics and dynamics