Into the storm: diving into the winds of the ultra-hot Jupiter WASP-76 b with HARPS and ESPRESSO

¹ Observatoire astronomique de l’Université de Genève, Chemin Pegasi 51b, 1290 Versoix, Switzerland
e-mail: julia.seidel@unige.ch
² Department of Physics, and Institute for Research on Exoplanets, Université de Montréal, Montréal, H3T 1J4, Canada
³ Lund Observatory, Box 43, Sölvegatan 27, 22100 Lund, Sweden
⁴ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Florence, Italy
⁵ Université Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
⁶ Instituto de Astrofísica e Ciências do Espaço, CAUP, Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal
⁷ Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal
⁸ Physikalisches Institut & NCCR PlanetS, Universität Bern, 3012 Bern, Switzerland
⁹ INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate (LC), Italy
¹⁰ Centro de Astrobiología (CAB, CSIC-INTA), Departamento de Astrofísica, ESAC campus 28692 Villanueva de la Cañada, Madrid, Spain
¹¹ INAF – Osservatorio Astronomico di Trieste, via Tiepolo 11, 34143 Trieste, Italy
¹² European Southern Observatory (ESO) – Alonso de Cordova 3107, Vitacura, Santiago, Chile
¹³ Instituto de Astrofísica de Canarias, Via Lactea sn, 38200, La Laguna, Tenerife, Spain
¹⁴ Centro de Astrofísica, Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal
¹⁵ Instituto de Astrofísica e Ciências do Espaço, Faculdade de Ciências da Universidade de Lisboa, Campo Grande, 1749-016 Lisboa, Portugal
¹⁶ INAF – Osservatorio Astrofisico di Torino, Strada Osservatorio, 20 10025 Pino Torinese (TO), Italy
¹⁷ Centro de Astrobiología (CSIC-INTA), Ctra. de Ajalvir km 4, 28850, Torrejón de Ardoz, Madrid, Spain
¹⁸ Institute for Fundamental Physics (IFPU), Via Beirut 2, 34151 Grignano TS, Italy
¹⁹ Departamento de Astrofísica, Universidad de La Laguna, 38200 San Cristobal de La Laguna, Spain
²⁰ Leiden Observatory, Leiden University, Postbus 9513, 2300 RA Leiden, The Netherlands

Received: 15 February 2021
Accepted: 20 July 2021

Abstract

Context. Despite swift progress in the characterisation of exoplanet atmospheres in composition and structure, the study of atmospheric dynamics has not progressed at the same speed. While theoretical models have been developed to describe the lower layers of the atmosphere, and independently, the exosphere, little is known about the intermediate layers up to the thermosphere.

Aims. We aim to provide a clearer picture of atmospheric dynamics for the class of ultra-hot Jupiters, which are highly irradiated gas giants, based on the example of WASP-76 b.

Methods. We jointly analysed two datasets that were obtained with the HARPS and ESPRESSO spectrographs to interpret the resolved planetary sodium doublet. We then applied the MERC code, which retrieves wind patterns, speeds, and temperature profiles on the line shape of the sodium doublet. An updated version of MERC, with added planetary rotation, also provides the possibility of modelling the latitude dependence of the wind patterns.

Results. We retrieve the highest Bayesian evidence for an isothermal atmosphere, interpreted as a mean temperature of 3389 ± 227 K, a uniform day- to nightside wind of 5.5_−2.0^+1.4 km s⁻¹ in the lower atmosphere with a vertical wind in the upper atmosphere of 22.7_−4.1^+4.9 km s⁻¹, switching atmospheric wind patterns at 10⁻³ bar above the reference surface pressure (10 bar).

Conclusions. Our results for WASP-76 b are compatible with previous studies of the lower atmospheric dynamics of WASP-76 b and other ultra-hot Jupiters. They highlight the need for vertical winds in the intermediate atmosphere above the layers probed by global circulation model studies to explain the line broadening of the sodium doublet in this planet. This work demonstrates the capability of exploiting the resolved spectral line shapes to observationally constrain possible wind patterns in exoplanet atmospheres. This is an invaluable input to more sophisticated 3D atmospheric models in the future.