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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.