Hot Exoplanet Atmospheres Resolved with Transit Spectroscopy (HEARTS)

III. Atmospheric structure of the misaligned ultra-hot Jupiter WASP-121b^★,★★

¹ Observatoire de l’Université de Genève, 51 chemin des Maillettes, 1290 Versoix, Switzerland
e-mail: vincent.bourrier@unige.ch
² Instituto de Astrofísica de Canarias (IAC), 38205 La Laguna, Tenerife, Spain
³ Leiden Observatory, Leiden University, Postbus 9513, 2300 RA Leiden, The Netherlands
⁴ Center for Space and Habitability, Universität Bern, Gesellschaftsstrasse 6, 3012 Bern, Switzerland
⁵ European Southern Observatory, Alonso de Córdova 3107, Vitacura, Región Metropolitana, Chile
⁶ Departamento de Matemática y Física Aplicadas, Universidad Católica de la Santísima Concepción, Alonso de Rivera 2850 Concepción, Chile
⁷ Departamento de Astrofísica, Universidad de La Laguna (ULL), 38206 La Laguna, Tenerife, Spain
⁸ INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁹ Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands

Received: 5 September 2019
Accepted: 19 December 2019

Abstract

Ultra-hot Jupiters offer interesting prospects for expanding our theories on dynamical evolution and the properties of extremely irradiated atmospheres. In this context, we present the analysis of new optical spectroscopy for the transiting ultra-hot Jupiter WASP-121b. We first refine the orbital properties of WASP-121b, which is on a nearly polar (obliquity ψ^North = 88.1 ± 0.25° or ψ^South = 91.11 ± 0.20°) orbit, and exclude a high differential rotation for its fast-rotating (P < 1.13 days), highly inclined (i_⋆^North = 8.1_−2.6^+3.0° or i_⋆^South = 171.9_−3.4^+2.5°) star. We then present a new method that exploits the reloaded Rossiter-McLaughlin technique to separate the contribution of the planetary atmosphere and of the spectrum of the stellar surface along the transit chord. Its application to HARPS transit spectroscopy of WASP-121b reveals the absorption signature from metals, likely atomic iron, in the planet atmospheric limb. The width of the signal (14.3 ± 1.2 km s⁻¹) can be explained by the rotation of the tidally locked planet. Its blueshift (−5.2 ± 0.5 km s⁻¹) could trace strong winds from the dayside to the nightside, or the anisotropic expansion of the planetary thermosphere.