  High-resolution optical spectroscopy is a powerful tool to characterise
exoplanetary atmospheres from the ground. The sodium D lines, with their large
cross sections, are especially suited to study the upper layers of atmospheres
in this context. We report on the results from HEARTS, a spectroscopic survey
of exoplanet atmospheres, performing a comparative study of hot gas giants to
determine the effects of stellar irradiation. In this second installation of
the series, we highlight the detection of neutral sodium on the ultra-hot giant
WASP-76b. We observed three transits of the planet using the HARPS
high-resolution spectrograph at the ESO 3.6m telescope and collected 175
spectra of WASP-76. We repeatedly detect the absorption signature of neutral
sodium 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 at
half maximum (FWHM) of $27.6\pm2.8$ km s$^{-1}$. This is significantly broader
than the line spread function of HARPS ($2.7$ km s$^{-1}$). We surmise that the
observed broadening could trace the super-rotation in the upper atmosphere of
this ultra-hot gas giant.
