Precise measurement of WASP-31 b’s Rossiter-McLaughlin effect and characterization of the planet transmission spectra^★

¹ Observatoire de l’Université de Genève, Chemin Pegasi 51, 1290 Versoix, Switzerland
² Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal
³ Département de Physique, Institut Trottier de Recherche sur les Exoplanètes, Université de Montréal, Montréal, Québec H3T 1J4, Canada
⁴ INAF-Astronomical Observatory, via Tiepolo 11, 34143 Trieste, Italy
⁵ Instituto de Astrofísica de Canarias (IAC), 38205 La Laguna, Tenerife, Spain
⁶ Departamento de Astrofísica, Universidad de La Laguna (ULL), 38206 La Laguna, Tenerife, Spain
⁷ Centro de Astrofísica da Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal
⁸ Zalando Switzerland, Hardstrasse 201, 8005 Zürich, Switzerland
⁹ Centro de Astrobiología, CSIC-INTA Camino Bajo del Castillo, s/n, 28692 Villanueva de la Cañada, Madrid, Spain
¹⁰ Institute of Space Sciences (ICE, CSIC), Carrer de Can Magrans S/N, Campus UAB, Cerdanyola del Valles 08193, Spain
¹¹ Institut d’Estudis Espacials de Catalunya (IEEC), 08860 Castelldefels (Barcelona), Spain
¹² Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal
¹³ Laboratoire Lagrange, Observatoire de la Côte d’Azur, CNRS, Université Côte d’Azur, Nice, France
¹⁴ European Southern Observatory, Alonso de Córdova 3107, Vitacura, Región Metropolitana, Chile
¹⁵ INAF – Osservatorio Astrofisico di Torino, via Osservatorio 20, 10025 Pino Torinese, Italy
¹⁶ Institute of Physics, École Polytechnique Federale de Lausanne (EPFL), Observatoire de Sauverny, Chemin Pegasi 51b, 1290 Versoix, Switzerland

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

Received: 4 June 2025
Accepted: 13 January 2026

Abstract

Context. Hot Jupiters are ideal natural laboratories to investigate atmospheric composition and dynamics. However, high-resolution transmission spectroscopy is currently limited by our capability of removing planet-occulted line-distortion (POLD) contamination from the signal.

Aims. In this paper, we aim to characterize the transmission spectrum of WASP-31 b from two and a half transits observed with the ESPRESSO spectrograph at the VLT.

Methods. The Rossiter-McLaughlin (RM) signature was analyzed using the RM “revolutions” method. Before extracting the transmission spectrum of the planet, we corrected the dataset for telluric lines using molecfit and further modeled the POLD deformations using EvE.

Results. We confirm the planet low sky-projected spin-orbit angle from previous studies and further refine its value to λ = −0.09_−0.32^+0.31 deg. We do not detect any species (including previously detected species such as K or CrH) in the planetary atmosphere. In most cases the non-detections are due to the strong POLDs contamination or lack of observable lines in the ESPRESSO wavelength range, and so previous detections cannot be ruled out.

Conclusions. Planet-occulted line-distortion contamination continues to be the main limitation of high-resolution transmission spectroscopy for species present in both the star and the planet, hindering atmospheric detections even with state-of-the-art models, in particular for planets with a low sky-projected spin-orbit angle. Developing advanced techniques to isolate planetary signatures is of utmost importance in the advent of ELT-like observations.