  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+ 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 H2O 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) 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) is broadly consistent with the value of solar abundance. In
addition, we included the equatorial rotation velocity (Veq ) in the retrieval
when analyzing the line profile broadening. The obtained Veq is 7.0 km/s for
WASP-18b and 5.2 km/s for WASP-76b, which are consistent with the tidally
locked rotational velocities.
