2025MNRAS.536.2581J

Atmospheric studies are essential for elucidating the formation history, evolutionary processes, and atmospheric dynamics of exoplanets. High-resolution transmission spectroscopy offers the advantage of detecting subtle variations in stellar spectral profiles, thereby enabling the identification of the sources of observed signals. In this study, we present the transmission spectra of the exoplanet WASP-77 A b, a hot Jupiter with a 1.36-d orbital period around a G8 host star with $V=11.29 \, {\rm mag}$. These observations were conducted using the high-resolution spectrograph ESPRESSO (Echelle SPectrograph for Rocky Exoplanets and Stable Spectroscopic Observations) at the VLT (Very Large Telescope) over three transit events. We analyse the Rossiter-McLaughlin effect for WASP-77 A and determine a projected spin-orbit angle of ${\lambda = 16.131^{\circ }}^{+2.106}_{-2.324}$, indicating that the planet's orbit is nearly aligned. Following the generation of transmission spectra for the three nights, we model and correct for centre-to-limb variation and the Rossiter-McLaughlin effects. In the residual transmission spectra, we detect ${\rm H} \, \alpha$, ${\rm H} \, \beta$, and Ca II H with a significance exceeding $3.5 \sigma$. After applying $0.1\!-\!0.5$ Å masks to the cores of these lines to mitigate stellar contamination, all of them still show visible absorptions although not significant, suggesting at least partial planet contribution to them. Therefore, we are yet unable to confirm or reject the planetary origin of these spectral signals based on the current data set. Further investigation of WASP-77 A b's atmosphere, particularly in areas beyond the terminator region, is essential to illuminate the planet's two-dimensional atmospheric structure.