Detection of Na I and Ha absorption in the atmosphere of MASCARA-2b/KELT-20b

We have used the HARPS-North high resolution spectrograph (R=115000) at TNG to observe one transit of the highly irradiated planet MASCARA-2b/KELT-20b. Using only one transit observation, we are able to resolve two spectral features centred on the atomic sodium (Na I) doublet position with line contrasts of 0.45+-0.09% (D2) and 0.43+-0.10% (D1), and one centred in Ha line with 0.6+-0.1% contrast. The transmission light curves have been also computed, showing a large Rossiter-McLaughlin effect, and consistent absorption depth values with those measured from the final transmission spectrum. While the S/N of the final transmission spectrum is insufficient to adjust different temperature profiles to the lines, we find that higher temperatures than the equilibrium (Teq=2260+-50K) are needed to explain the lines contrast. Particularly, we find that the Na I lines core require T=4400+-561K. MASCARA-2b, like other planets orbiting A-type stars, receives a large amount of UV energy from its host star. This energy excites the atomic hydrogen and produces Ha absorption, leading to the expansion and ablation of the atmosphere. The study of other Balmer lines in the transmission spectrum would allow the determination of the atmospheric temperature profile and the lifetime of the atmosphere with escape rate measurements. In the case of MASCARA-2b, strong residuals have been observed in the Hb and Hg lines, but the co-addition of more transit observations is needed to figure out their true origin.

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