The atmospheric composition of the ultra-hot Jupiter WASP-178 b observed with ESPRESSO

Context. Ultra-hot Jupiters (UHJ) have emerged as ideal testbeds for new techniques for studying exoplanet atmospheres. Only a limited number of them are currently well studied, however. Aims. We search for atmospheric constituents for the UHJ WASP-178 b with two ESPRESSO transits. Additionally, we show parallel photometry that we used to obtain updated and precise stellar, planetary, and orbital parameters.<br /> Methods. The two transits we obtained were analysed with narrow-band transmission spectroscopy and with the cross-correlation technique to provide detections at different altitude levels. We focused on searching for Na I, H alpha, H beta, H gamma, Mg I, and Li I lines in narrow-band data, as well as Fe I and Fe II, and attempted to confirm Mg I with the cross-correlation technique. We corrected for the Rossiter-McLaughlin effect and regions with a low signal-to-noise ratio due to Na I absorption in the interstellar medium. We then verified our results via bootstrapping.<br /> Results. We report the resolved line detections of Na I (5.5 sigma and 5.4 sigma), H alpha (13 sigma), H beta (7.1 sigma), and tentatively Mg I (4.6 sigma). With a cross-correlation, we confirm the Mg I detection (7.8 sigma and 5.8 sigma), and we additionally report the detections of Fe I (12 sigma and 10 sigma) and Fe II (11 sigma and 8.4 sigma) on both nights separately. The detection of Mg I remains tentative, however, because the results on the two nights differ. The results also differ compared with the properties derived from the narrow-band data.<br /> Conclusions. None of our resolved spectral lines probing the middle to upper atmosphere shows significant shifts relative to the planetary rest frame. H alpha and H beta exhibit a respective line broadening of 39.6 +/- 2.1 km s(-1) and 27.6 +/- 4.6 km s(-1), however, indicating the onset of possible escape. WASP-178 b differs from similar UHJ by its lack of strong atmospheric dynamics in the upper atmosphere. The broadening seen for Fe I (15.66 +/- 0.58 km s(-1)) and Fe II (11.32 +/- 0.52 km s(-1)) might indicate the presence of winds in the mid-atmosphere, however. Future studies of the impact of the flux variability caused by the host star activity might shed more light on the subject. Previous work indicated the presence of SiO cloud-precursors in the atmosphere of WASP-178 b and a lack of Mg I and Fe II. However, our results suggest that a scenario in which the planetary atmosphere is dominated by Mg I and Fe II is more likely. In light of our results, we encourage future observations to further elucidate these atmospheric properties.