Planets as probes of stellar atmospheres: Characterizing local spectra via Doppler Shadow

As the number of known exoplanets grows, the focus has shifted toward characterizing these worlds and their atmospheres. However, the detection of exoplanet atmospheres is challenged by our knowledge of stellar physics, including activity and center-to-limb variations (CLV), as the spectrum is not homogeneous across the apparent surface of the star. These inhomogeneities alter the profile of spectral lines and complicate transmission spectroscopy atmospheric retrievals, so understanding stellar surfaces is crucial for our success. To solve this problem, we can use the “Doppler Shadow” method, which allows us to study local regions of the stellar surface, using the planet as a probe. This technique provides insights into stellar surface physics and how the spectral lines themselves change in function of limb position, as well as allow us to better understand the behavior of the distortions in transmission spectra. Currently, the Doppler Shadow technique is the only one that allows to obtain local spectra for stars other than the Sun, and it has so far been applied only on customized line-lists (CCFs). New-generation instruments like ESPRESSO, in the optical, offer unprecedented spectral resolution and stability, enabling revisiting and expanding this technique to a much wider range of planetary systems. In this poster, we present novel results regarding the application of the Doppler Shadow technique to ESPRESSO observations of HD 189733 and HD 209458. We will explore the effect of spectral type in the Doppler Shadow retrieval, as well as its capability in extracting both local individual spectral lines and local CCFs. These results were enabled by HECATE, a robust and flexible analysis pipeline capable of extracting spectra occulted by transiting planets. This tool introduces automation, significantly improving reproducibility and scalability, and is fully documented and publicly available.