Detection of a high-velocity sodium feature on the ultra-hot Jupiter WASP-121 b^★

¹ European Southern Observatory, Alonso de Córdova 3107, Vitacura, Región Metropolitana, Chile
e-mail: jseidel@eso.org
² INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate (LC), Italy
³ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Florence, Italy
⁴ Observatoire astronomique de l’Université de Gèneve, Chemin Pegasi 51b, 1290 Versoix, Switzerland
⁵ Instituto de Astrofísica de Canarias, Via Lactea sn, 38200 La Laguna, Tenerife, Spain
⁶ Centro de Astrobiología (CSIC-INTA), Ctra. de Ajalvir km 4, 28850 Torrejón de Ardoz, Madrid, Spain
⁷ Physikalisches Institut & NCCR PlanetS, Universität Bern, 3012 Bern, Switzerland
⁸ Department of Physics, and Trottier Institute for Research on Exoplanets, Université de Montréal, Montréal H3T 1J4, Canada
⁹ INAF – Osservatorio Astronomico di Trieste, via Tiepolo 11, 34143 Trieste, Italy
¹⁰ Instituto de Astrofísica e Ciências do Espaço, CAUP, Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal
¹¹ Centro de Astrofísica da Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal
¹² Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal
¹³ Institute for Fundamental Physics (IFPU), Via Beirut 2, 34151 Grignano TS, Italy
¹⁴ Instituto de Astrofísica e Ciências do Espaço, Faculdade de Ciências da Universidade de Lisboa, Campo Grande, 1749-016 Lisboa, Portugal
¹⁵ INAF – Osservatorio Astrofisico di Torino, Strada Osservatorio, 20 10025 Pino Torinese (TO), Italy

Received: 25 December 2022
Accepted: 10 March 2023

Abstract

Context. Ultra-hot Jupiters, with their high equilibrium temperatures and resolved spectral lines, have emerged as a perfect testbed for new analysis techniques in the study of exoplanet atmospheres. In particular, the resolved sodium doublet as a resonant line has proven a powerful indicator to probe the atmospheric structure over a wide pressure range.

Aims. We aim to explore an atmospheric origin of the observed blueshifted feature next to the sodium doublet of the ultra-hot Jupiter WASP-121 b using a partial transit obtained with the 4-UT mode of ESPRESSO. We intend to study its atmospheric dynamics visible across the terminator by splitting the data into mid-transit and egress.

Methods. We explored the impact of the Rossiter-McLaughlin effect on the line shape of the sodium doublet. The partial transit is separated into one dataset centred around mid-transit and one dataset comprising the second part of the transit and egress. Lastly, the atmospheric retrieval code, Multinested Eta Retrieval Code (MERC), was applied to both datasets in order to study the imprint of atmospheric dynamics on the line shape of the sodium doublet.

Results. We determine that the blueshifted high-velocity absorption component is generated only during the egress part of the transit when a larger fraction of the day side of the planet is visible. For the egress data, MERC retrieves the blueshifted high-velocity absorption component as an equatorial day-to-night side wind across the evening limb, with no zonal winds visible on the morning terminator with weak evidence compared to a model with only vertical winds. For the mid-transit data, the observed line broadening is attributed to a vertical, radial wind.

Conclusions. We attribute the equatorial day-to-night-side wind over the evening terminator to a localised jet and restrain its existence between the substellar point and up to 10° to the terminator in longitude, an opening angle of the jet of at most 60° in latitude, and a lower boundary in altitude between [1.08, 1.15] R_p. As a hypothesis, we propose that the jet is produced by the excitation of standing planetary scale Rossby waves by stellar irradiation and subsequently broken by Kelvin-Helmholtz instabilities. Due to the partial nature of the transit, we cannot make any statements on whether the jet is truly super-rotational and one-sided or part of a symmetric day-to-night-side atmospheric wind from the hotspot.