Wyttenbach, A.A.WyttenbachLovis, C.C.LovisEhrenreich, D.D.EhrenreichBourrier, V.V.BourrierPino, LorenzoLorenzoPinoAllart, R.R.AllartAstudillo-Defru, N.N.Astudillo-DefruCegla, H. M.H. M.CeglaHeng, K.K.HengLavie, B.B.LavieMelo, C.C.MeloMurgas, F.F.MurgasSanterne, A.A.SanterneSégransan, D.D.SégransanUdry, S.S.UdryPepe, F.F.Pepe2022-04-012022-04-0120170004-6361http://hdl.handle.net/20.500.12386/32076High-resolution optical spectroscopy during the transit of HD 189733b, a prototypical hot Jupiter, allowed the resolution of the Na I D sodium lines in the planet, giving access to the extreme conditions of the planet upper atmosphere. We have undertaken HEARTS, a spectroscopic survey of exoplanet upper atmospheres, to perform a comparative study of hot gas giants and determine how stellar irradiation affect them. Here, we report on the first HEARTS observations of the hot Saturn-mass planet WASP-49b. We observed the planet with the HARPS high-resolution spectrograph at ESO 3.6 m telescope. We collected 126 spectra of WASP-49, covering three transits of WASP-49b. We analyzed and modeled the planet transit spectrum, while paying particular attention to the treatment of potentially spurious signals of stellar origin. We spectrally resolve the Na I D lines in the planet atmosphere and show that these signatures are unlikely to arise from stellar contamination. The large contrasts of 2.0 ± 0.5% (D<SUB>2</SUB>) and 1.8 ± 0.7% (D<SUB>1</SUB>) require the presence of hot neutral sodium ( K) at high altitudes ( 1.5 planet radius or 45 000 km). From estimating the cloudiness index of WASP-49b, we determine its atmosphere to be cloud free at the altitudes probed by the sodium lines. WASP-49b is close to the border of the evaporation desert and exhibits an enhanced thermospheric signature with respect to a farther-away planet such as HD 189733b. <P />Based on observations made at ESO 3.6 m telescope at the La Silla Observatory under ESO program 096.C-0331....STAMPAenArticle10.1051/0004-6361/2016300632-s2.0-85020019579http://arxiv.org/abs/1702.00448v1https://www.aanda.org/articles/aa/full_html/2017/06/aa30063-16/aa30063-16.html2017A&A...602A..36WFIS/05 - ASTRONOMIA E ASTROFISICA