Carbon monoxide emission lines reveal an inverted atmosphere in the ultra hot Jupiter WASP-33 b consistent with an eastward hot spot

We report the first detection of CO emission at high spectral resolution in day-side infrared thermal spectrum an exoplanet. These lines, found atmosphere transiting ultra hot Jupiter (UHJ) WASP-33 b, provide unambiguous evidence its inversion. Using spectra from MMT Exoplanet Atmosphere Survey (MEASURE, $R\sim15,000$), covering pre- and post-eclipse phases, we cross-correlate with 1D PHOENIX templates to detect S/N = 7.9 ($v_{sys}=0.15^{+0.64}_{-0.65}$ km/s, $K_{p}=229.5^{+1.1}_{-1.0}$ km/s). Moreover, using cross-correlation-to-log-likelihood mapping, find that scaling parameter which controls line contrast changes phase. thus use general circulation model SPARC/MITgcm post-processed by 3D gCMCRT radiative transfer code interpret this variation, finding it consistent eastward-shifted spot. Pre-eclipse, when spot faces Earth, profiles are shallower leading smaller despite greater overall flux. Post-eclipse, western part Earth has much steeper profiles, larger less This demonstrates within log-likelihood framework, even relatively moderate can be used understand nature close-in exoplanets, traded for photon-collecting power induced Doppler-shift is sufficiently large. highlight as a good probe UHJ structure dynamics does not suffer stellar activity, unlike species also present host star e.g. iron lines.