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

International audienceHigh‐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‐2) and 1.8 +/‐ 0.7% (D‐1) require the presence of hot neutral sodium (2950(‐500)(+400) K) at high altitudes (similar to 1.5 planet radius or similar to 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