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High-resolution optical spectroscopy during the transit of HD 189733b, aprototypical hot Jupiter, allowed the resolution of the Na I D sodium lines inthe planet, giving access to the extreme conditions of the planet upperatmosphere. We have undertaken HEARTS, a spectroscopic survey of exoplanetupper atmospheres, to perform a comparative study of hot gas giants anddetermine how stellar irradiation affect them. Here, we report on the firstHEARTS observations of the hot Saturn-mass planet WASP-49b. We observed theplanet with the HARPS high-resolution spectrograph at ESO 3.6m telescope. Wecollected 126 spectra of WASP-49, covering three transits of WASP-49b. Weanalyzed and modeled the planet transit spectrum, while paying particularattention to the treatment of potentially spurious signals of stellar origin.We spectrally resolve the Na I D lines in the planet atmosphere and show thatthese signatures are unlikely to arise from stellar contamination. The largecontrasts of $2.0\pm0.5\%$ (D$_2$) and $1.8\pm0.7\%$ (D$_1$) require thepresence of hot neutral sodium ($2,950^{+400}_{-500}$ K) at high altitudes($\sim$1.5 planet radius or $\sim$45,000 km). From estimating the cloudinessindex of WASP-49b, we determine its atmosphere to be cloud free at thealtitudes probed by the sodium lines. WASP-49b is close to the border of theevaporation desert and exhibits an enhanced thermospheric signature withrespect to a farther-away planet such as HD 189733b.

Hot Exoplanet Atmospheres Resolved with Transit Spectroscopy (HEARTS) I. Detection of hot neutral sodium at high altitudes on WASP-49b