Retrieving wind properties from the ultra-hot dayside of WASP-189 b with CRIRES⁺

¹ Institut für Astrophysik und Geophysik, Georg-August-Universität, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
² Universitäts-Sternwarte, Fakultät für Physik, Ludwig-Maximilians-Universität München, Scheinerstr. 1, 81679 München, Germany
³ Exzellenzcluster Origins, Boltzmannstraße 2, 85748 Garching, Germany
⁴ Department of Astronomy, University of Science and Technology of China, Hefei 230026, China
⁵ Department of Physics and Astronomy, Uppsala University, Box 516, 75120 Uppsala, Sweden
⁶ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
⁷ Institut de Recherche en Astrophysique et Planétologie, Université de Toulouse, CNRS, IRAP/UMR 5277, 14 avenue Edouard Belin, 31400 Toulouse, France
⁸ Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
⁹ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
¹⁰ Instituto de Astrofísica de Andalucía – CSIC, Glorieta de la Astronomía s/n, 18008 Granada, Spain

^★ Corresponding author; fabio.lesjak@uni-goettingen.de

Received: 5 July 2024
Accepted: 21 November 2024

Abstract

Context. The extreme temperature gradients from day- to nightside in the atmospheres of hot Jupiters generate fast winds in the form of equatorial jets or day-to-night flows. Observations of blue-shifted and red-shifted signals in the transmission and dayside spectra of WASP-189 b have sparked discussions about the nature of winds on this planet.

Aims. To investigate the structure of winds in the atmosphere of the ultra-hot Jupiter WASP-189 b, we studied its dayside emission spectrum with CRIRES⁺ in the spectral K band.

Methods. After removing stellar and telluric lines, we used the cross-correlation method to search for a range of molecules and detected emission signals of CO and Fe. Subsequently, we employed a Bayesian framework to retrieve the atmospheric parameters relating to the temperature–pressure structure and chemistry, and incorporated a numerical model of the line profile influenced by various dynamic effects to determine the wind structure.

Results. The cross-correlation signals of CO and Fe showed a velocity offset of ~6 km s⁻¹, which could be caused by a fast day-tonight wind in the atmosphere of WASP-189 b. The atmospheric retrieval showed that the line profile of the observed spectra is best fitted by the presence of a day-to-night wind of 4.4_−2.2^+1.8 km s⁻¹, while the retrieved equatorial jet velocity of 1.0_−1.8^+0.9 km s⁻¹ is consistent with the absence of such a jet. Such a wind pattern is consistent with the observed line broadening and can explain the majority of the velocity offset, while uncertainties in the ephemerides and the effects of a hot spot could also contribute to this offset. We further retrieved an inverted temperature-pressure profile, and under the assumption of equilibrium chemistry we retrieved a C/O ratio of 0.32_−0.14^+0.41 and a metallicity of M/H = 1.40_−0.60^+1.39.

Conclusions. We showed that red-shifts of a few km s⁻¹ in the dayside spectra could be explained by day-to-night winds. Further studies combining transmission and dayside observations could advance our understanding of WASP-189 b’s atmospheric circulation by improving the uncertainties in the velocity offset and wind parameters.