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

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.