The ESPRESSO transmission spectrum of HD189733 b

Extracting the planetary sodium and lithium signatures amid stellar contamination^★

¹ Geneva Observatory, University of Geneva, Chemin Pegasi 51b, 1290 Versoix, Switzerland
² Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
³ Departamento de Fisica e Astronomia, Faculdade de Ciencias, Universidade do Porto, Rua Campo Alegre, 4169-007 Porto, Portugal
⁴ Institute of Space Sciences (ICE, CSIC), Carrer de Can Magrans S/N, Campus UAB, Cerdanyola del Valles 08193, Spain
⁵ Institut d’Estudis Espacials de Catalunya (IEEC), 08860 Castellde-fels (Barcelona), Spain
⁶ Département de Physique, Institut Trottier de Recherche sur les Exoplanètes, Université de Montréal, Montréal, Québec, H3T 1J4, Canada
⁷ European Southern Observatory, Alonso de Córdova 3107, Vitacura, Región Metropolitana, Chile
⁸ Laboratoire Lagrange, Observatoire de la Côte d’Azur, Nice, France
⁹ Centro de Astrobiología, CSIC-INTA, Camino Bajo del Castillo s/n, 28692 Villanueva de la Canada, Madrid, Spain
¹⁰ INAF – Osservatorio Astrofisico di Trieste, via G. B. Tiepolo 11, 34143 Trieste, Italy
¹¹ IFPU-Institute for Fundamental Physics of the Universe, via Beirut 2, 34151 Trieste, Italy
¹² Center for Space and Habitability, University of Bern, Gesellschaftsstrasse 6, 3012 Bern, Switzerland
¹³ Physics Institute of University of Bern, Gesellschaftsstrasse 6, CH3012 Bern, Switzerland
¹⁴ Instituto de Astrofísica de Canarias, 38205 La Laguna, Tenerife, Spain
¹⁵ Universidad de La Laguna, Dept. Astrofísica, 38206 La Laguna, Tenerife, Spain
¹⁶ Centro de Astrofísica da Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal
¹⁷ Observatoire François-Xavier Bagnoud - OFXB, 3961 Saint-Luc, Switzerland
¹⁸ INAF – Osservatorio Astrofisico di Torino, Via Osservatorio 20, I10025 Pino Torinese, Italy

^★★ Corresponding author: dany.mounzer@unige.ch

Received: 27 March 2025
Accepted: 17 June 2025

Abstract

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.