Hot Exoplanet Atmospheres Resolved with Transit Spectroscopy (HEARTS)

Wyttenbach, A.; Lovis, C.; Ehrenreich, D.; Bourrier, V.; Pino, L.; Allart, R.; Astudillo-Defru, N.; Cegla, H. M.; Heng, Kevin; Lavie, B.; Melo, C.; Murgas, F.; Santerne, A.; Ségransan, D.; Udry, S.; Pepe, F. (2017). Hot Exoplanet Atmospheres Resolved with Transit Spectroscopy (HEARTS). Astronomy and astrophysics, 602(A36), A36. EDP Sciences 10.1051/0004-6361/201630063

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High-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 3.6m 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% (D₂) and 1.8±0.7% (D₁) require the presence of hot neutral sodium (2,950+400−500 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.

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