Transmission spectroscopy of the ultra-hot Jupiter MASCARA-4 b Disentangling the hydrostatic and exospheric regimes of ultra-hot Jupiters

Context. Ultra-hot Jupiters (UHJs), rendering the hottest planetary atmospheres, offer great opportunities of detailed characterisation with high-resolution spectroscopy. MASCARA-4 b is a recently discovered close-in gas giant belonging to this category. Aims. We aim to characterise MASCARA-4 b, search for chemical species in its atmosphere, and put these in the context of the growing knowledge on the atmospheric properties of UHJs. Methods. In order to refine system and planet parameters, we carried out radial velocity measurements and transit photometry with the CORALIE spectrograph and EulerCam at the Swiss 1.2m Euler telescope. We observed two transits of MASCARA-4 b with the high-resolution spectrograph ESPRESSO at ESO's Very Large Telescope. We searched for atomic, ionic, and molecular species via individual absorption lines and cross-correlation techniques. These results are compared to literature studies on UHJs characterised to date. Results. With CORALIE and EulerCam observations, we update the mass of MASCARA-4 b (M-p = 1.675 +/- 0.241 M-Jup) as well as other system and planet parameters. In the transmission spectrum derived from ESPRESSO observations, we resolve excess absorption by H alpha, H beta, Na I D1 &D2, Ca II H&K, and a few strong lines of Mg I, Fe I, and Fe II. We also present the cross-correlation detection of Mg I, Ca I, Cr I, Fe I, and Fe II. The absorption strength of Fen significantly exceeds the prediction from a hydrostatic atmospheric model, as commonly observed in other UHJs. We attribute this to the presence of Fe II in the exosphere due to hydrodynamic outflows. This is further supported by the positive correlation of absorption strengths of Fe II with the Ha line, which is expected to probe the extended upper atmosphere and the mass loss process. Comparing transmission signatures of various species in the UHJ population allows us to disentangle the hydrostatic regime (as traced via the absorption by Mg I and Fe I) from the exospheres (as probed by H alpha and Fe II) of the strongly irradiated atmospheres.