Exploring the surface of HD 189733 via Doppler shadow analysis of planetary transits

¹ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
² Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal
³ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
⁴ Département de Physique, Institut Trottier de Recherche sur les Exoplanètes, Université de Montréal, Montréal, Québec H3T 1J4, Canada
⁵ Observatoire Astronomique de l’Université de Genève, Chemin Pegasi 51, Sauverny 1290, Switzerland

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it. ; This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 10 October 2025
Accepted: 17 February 2026

Abstract

Context. Transmission spectroscopy has advanced our understanding of exoplanet atmospheres, but it can be hindered by contamination originating in stellar heterogeneities, mainly coming from line-of-sight effects. Therefore, probing how stellar spectra vary across the stellar surface is essential to accurately disentangling stellar and planetary spectral contributions in transit observations. Such observations can actually be used to reconstruct the local stellar spectra behind the planet’s transit chord. These methodologies can help us learn more about the physics of stellar surface and how to tackle line-of-sight effects.

Aims. In this paper, we study the centre-to-limb variations of line profiles across the surface of HD 189733 using the ESPRESSO spectrograph. We build on other works by analysing the same sets of Fe I lines, allowing for a direct comparison of results and an assessment of the feasibility of applying the Doppler shadow technique with ESPRESSO. We gain a better understanding of the variations in line profiles, while also making a comparison between the data of HD 189733 and synthetic spectra and solar data.

Methods. We analysed spectra collected by ESPRESSO during two transits of HD 189733 b as separate sets of data. We performed a cross-correlation of each individual spectrum with two different masks made of selected Fe I spectral lines for a total of four sets of cross-correlation functions (CCFs) and employed a Doppler shadow methodology to retrieve profiles for local regions of the stellar surface. We then compared the results with previous works and with solar disc-resolved observations from IAG ATLAS. Finally, we compared the data with two separate transit simulations made using SOAPv4 with Turbospectrum synthetic spectra computed with MARCS stellar atmosphere models under local thermal equilibrium (LTE) and non-LTE (NLTE) conditions.

Results. For the profile depth of three sets of Fe I CCFs, we verified a statistically significant increase in line depth from the stellar limb to the centre. This variation was expected from simulations with MARCS models, although the solar data present a smaller gradient in the variation of line depth. In the case of the width of the line profiles, we verified that the profile width decreases from stellar limb to stellar centre for a set of Fe I CCFs. This result is consistent with the behaviour observed in solar data, but not reproduced by the simulations.

Conclusions. These results highlight the abilities of ESPRESSO in providing the necessary precision and resolution to study centre-to-limb variations of spectral line profiles on the surface of other stars with the use of CCFs. The local CCF profiles of HD 189733 agree with the IAG ATLAS data, but disagree with simulations on line widths, indicating that important physical processes are missing and must be included to recover accurate profile widths.