CRIRES<SUP>+</SUP> detection of CO emissions lines and temperature inversions on the dayside of WASP-18b and WASP-76b

The dayside atmospheres of ultra-hot Jupiters (UHJs) are predicted to possess temperature inversion layers with extremely high temperatures at high altitudes. We observed the dayside thermal emission spectra of WASP-18b and WASP-76b with the new CRIRES<SUP>+</SUP> high-resolution spectrograph at near-infrared wavelengths. Using the cross-correlation technique, we detected strong CO emission lines in both planets, which confirms the existence of temperature inversions on their dayside hemispheres. The two planets are the first UHJs orbiting F-type stars with CO emission lines detected; previous detections were mostly for UHJs orbiting A-type stars. Evidence of weak H<SUB>2</SUB>O emission signals is also found for both planets. We further applied forward-model retrievals on the detected CO lines and retrieved the temperature-pressure profiles along with the CO volume mixing ratios. The retrieved logarithmic CO mixing ratio of WASP-18b (−2.2<SUB>−1.5</SUB><SUP>+1.4</SUP>) is slightly higher than the value predicted by the self-consistent model assuming solar abundance. For WASP-76b, the retrieved CO mixing ratio (−3.6<SUB>−1.6</SUB><SUP>+1.8</SUP>) is broadly consistent with the value of solar abundance. In addition, we included the equatorial rotation velocity (υ<SUB>eq</SUB>) in the retrieval when analyzing the line profile broadening. The obtained υ<SUB>eq</SUB> is 7.0 ± 2.9 km s<SUP>−1</SUP> for WASP-18b and 5.2<SUB>−3.0</SUB><SUP>+2.5</SUP> km s<SUP>−1</SUP> for WASP-76b, which are consistent with the tidally locked rotational velocities.