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High-resolution optical spectroscopy is a powerful tool to characteriseexoplanetary atmospheres from the ground. The sodium D lines, with their largecross sections, are especially suited to study the upper layers of atmospheresin this context. We report on the results from HEARTS, a spectroscopic surveyof exoplanet atmospheres, performing a comparative study of hot gas giants todetermine the effects of stellar irradiation. In this second installation ofthe series, we highlight the detection of neutral sodium on the ultra-hot giantWASP-76b. We observed three transits of the planet using the HARPShigh-resolution spectrograph at the ESO 3.6m telescope and collected 175spectra of WASP-76. We repeatedly detect the absorption signature of neutralsodium in the planet atmosphere ($0.371\pm0.034\%$; $10.75 \sigma$ in a $0.75$\r{A} passband). The sodium lines have a Gaussian profile with full width athalf maximum (FWHM) of $27.6\pm2.8$ km s$^{-1}$. This is significantly broaderthan the line spread function of HARPS ($2.7$ km s$^{-1}$). We surmise that theobserved broadening could trace the super-rotation in the upper atmosphere ofthis ultra-hot gas giant.

Hot Exoplanet Atmospheres Resolved with Transit Spectroscopy (HEARTS) - II. A broadened sodium feature on the ultra-hot giant WASP-76b