2025A&A...700A..41M

Context. While transmission spectroscopy has allowed us to detect many atomic and molecular species in exoplanet atmospheres, the improvement in resolution and signal-to-noise ratio (S/N) enabled us to become sensitive to planet-occulted line distortions (POLDs) in the spectrum that are induced by center-to-limb variations (CLV) and the Rossiter-McLaughlin effect (RM). POLDs can bias the interpretation of the transmission spectrum, and it is hard to correct for them with stellar models.
Aims. We analyzed two ESPRESSO transits (R ∼ 140 000) of the archetypal hot Jupiter HD 189733 b. The transmission spectrum of this aligned system is heavily affected by POLDs, stellar activity, and instrumental effects. It is therefore a challenging study case of how to account for these effects when the planetary signal is retrieved from chemical species through transmission spectroscopy.
Methods. We used the ANTARESS workflow to process the datasets to ensure an accurate correction for telluric and instrumental contamination. With improved architectural parameters derived using the RM revolutions technique, we tested several methods of including and correcting the strong POLDs in the transmission spectrum. We then derived the absorption spectrum from sodium through simultaneous forward-modeling of the star and planet using the code called evaporating exoplanets (EvE).
Results. We confirm the previous detections of the sodium doublet signature in the upper atmosphere of HD 189733 b. When we accounted for POLDs and isolated the planetary signal from uncorrected stellar residuals, we found a shallower (0.432 ± 0.027%) and more strongly blueshifted (-7.97 ± 0.28 km s^–1) signal. We attempted to reinterpret the other high-resolution sodium studies of this system in light of our results. We suggest that the POLDs and stellar activity are insufficiently corrected for in all analyses, including ours. We also detected a planetary lithium signature of 0.102 ± 0.016% (6.4σ) at a blueshift of -2.4 ± 1.8 km s^–1.
Conclusions. We have probably reached limitations in the accuracy of theoretical stellar spectra and in our understanding of stellar variability at the timescale of a transit because we are unable to fully correct for the effect of POLDs in HD 189733 b transmission spectra. As we shift toward a new generation of ground-based spectrographs on the ELT with an even higher S/N and resolution, addressing these issues will be paramount for a proper characterization of exoplanet atmospheres with transit spectroscopy.