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. 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 <ns0:sc>I</ns0:sc>, Hα, Hβ, Hγ, Mg <ns0:sc>I</ns0:sc>, and Li <ns0:sc>I</ns0:sc> lines in narrow-band data, as well as Fe <ns0:sc>I</ns0:sc> and Fe <ns0:sc>II</ns0:sc>, and attempted to confirm Mg <ns0:sc>I</ns0:sc> with the cross-correlation technique. We corrected for the Rossiter-McLaughlin effect and regions with a low signal-to-noise ratio due to Na <ns0:sc>I</ns0:sc> absorption in the interstellar medium. We then verified our results via bootstrapping. Results. We report the resolved line detections of Na <ns0:sc>I</ns0:sc> (5.5σ and 5.4σ), Hα (13σ), Hβ (7.1σ), and tentatively Mg <ns0:sc>I</ns0:sc> (4.6σ). With a cross-correlation, we confirm the Mg <ns0:sc>I</ns0:sc> detection (7.8 σ and 5.8 σ), and we additionally report the detections of Fe <ns0:sc>I</ns0:sc> (12σ and 10σ) and Fe <ns0:sc>II</ns0:sc> (11σ and 8.4σ) on both nights separately. The detection of Mg <ns0:sc>I</ns0:sc> 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. Conclusions. None of our resolved spectral lines probing the middle to upper atmosphere shows significant shifts relative to the planetary rest frame. Hα and Hβ 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 <ns0:sc>I</ns0:sc> (15.66 ± 0.58 km s−1) and Fe <ns0:sc>II</ns0:sc> (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 <ns0:sc>I</ns0:sc> and Fe <ns0:sc>II</ns0:sc>. However, our results suggest that a scenario in which the planetary atmosphere is dominated by Mg <ns0:sc>I</ns0:sc> and Fe <ns0:sc>II</ns0:sc> is more likely. In light of our results, we encourage future observations to further elucidate these atmospheric properties.