J/A+A/634/L4ivo://CDS.VizieR/J/A+A/634/L4doi:10.26093/cds/vizier.36349004CDSdos Santos L.A.Ehrenreich D.Bourrier V.Astudillo-Defru N.,Bonfils X.Forget F.Lovis C.Pepe F.Udry S2020-04-29T07:18:22Z2020-01-31T09:40:44ZCDS support team

CDS, Observatoire de Strasbourg, 11 rue de l'Universite, F-67000 Strasbourg, France

cds-question@unistra.frm-starsexoplanetsspectroscopyultraviolet-astronomyK2-18 b is a transiting mini-Neptune that orbits a nearby (38pc), 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-{alpha} transit spectroscopy with the Space Telescope Imaging Spectrograph (STIS) instrument installed on the Hubble Space Telescope (HST). We analyzed the time-series of fluxes of the stellar Lyman-{alpha} 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-{alpha} emission of K2-18, we estimate an EUV irradiation in the range 10^1^-10^2^erg/s/cm^2^ and a total escape rate on the order of 10^8^g/s. 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.2020A&A...634L...4Dhttps://cdsarc.cds.unistra.fr/viz-bin/cat/J/A+A/634/L4CatalogResearchIsServedByTAP VizieR generic servicehttps://cds.unistra.fr/vizier-org/licences_vizier.htmlhttps://vizier.cds.unistra.fr/viz-bin/VizieR-2?-source=J/A+A/634/L4https://vizier.iucaa.in/viz-bin/VizieR-2?-source=J/A+A/634/L4http://vizieridia.saao.ac.za/viz-bin/VizieR-2?-source=J/A+A/634/L4https://vizier.cds.unistra.fr/viz-bin/votable?-source=J/A+A/634/L4https://vizier.iucaa.in/viz-bin/votable?-source=J/A+A/634/L4http://vizieridia.saao.ac.za/viz-bin/votable?-source=J/A+A/634/L4GETtext/xml+votablehttps://tapvizier.cds.unistra.fr/TAPVizieR/tapUVdefaultJ/A+A/634/L4/orbit1Spectrum taken in orbit 1recnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordintlambdaCentral wavelength of the pixel0.1nmem.wl.central;instr.pixeldoubleFluxFlux density in the pixel (in erg/cm^2^/s/{AA})10mW.m**-2.nm**-1phot.fluxdoublee_FluxFlux density uncertainty in the pixel (in erg/cm^2^/s/{AA})10mW.m**-2.nm**-1stat.error;phot.fluxdoubleJ/A+A/634/L4/orbit2Spectrum taken in orbit 2recnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordintlambdaCentral wavelength of the pixel0.1nmem.wl.central;instr.pixeldoubleFluxFlux density in the pixel (in erg/cm^2^/s/{AA})10mW.m**-2.nm**-1phot.fluxdoublee_FluxFlux density uncertainty in the pixel (in erg/cm^2^/s/{AA})10mW.m**-2.nm**-1stat.error;phot.fluxdoubleJ/A+A/634/L4/orbit3Spectrum taken in orbit 3recnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordintlambdaCentral wavelength of the pixel0.1nmem.wl.central;instr.pixeldoubleFluxFlux density in the pixel (in erg/cm^2^/s/{AA})10mW.m**-2.nm**-1phot.fluxdoublee_FluxFlux density uncertainty in the pixel (in erg/cm^2^/s/{AA})10mW.m**-2.nm**-1stat.error;phot.fluxdoubleJ/A+A/634/L4/orbit4Spectrum taken in orbit 4recnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordintlambdaCentral wavelength of the pixel0.1nmem.wl.central;instr.pixeldoubleFluxFlux density in the pixel (in erg/cm^2^/s/{AA})10mW.m**-2.nm**-1phot.fluxdoublee_FluxFlux density uncertainty in the pixel (in erg/cm^2^/s/{AA})10mW.m**-2.nm**-1stat.error;phot.fluxdouble