Carbon monoxide emission lines indicate an offset hotspot in the inverted atmosphere of the ultra hot Jupiter WASP-33 b

We report the first detection of CO emission lines at high spectral resolution in the thermal spectrum of an exoplanet atmosphere. These emission lines, seen in the spectrum of the ultra hot Jupiter WASP-33 b, provide unambiguous evidence of a thermal inversion layer in its atmosphere, and are the first results from the MMT Exoplanet Atmosphere Survey (MEASURE) using ARIES with the adaptive secondary mirror at the 6.5-m MMT. Moreover, by incorporating these R=15,000 spectra into a Bayesian framework with 1D PHOENIX atmospheric models, we show via Cross-Correlation-to-log-Likelihood mapping that the spectra indicate an offset hotspot. This arises as the post-eclipse spectra favour a scaling parameter that creates shallower spectral line models than those for pre-eclipse, indicating variation due to an hotspot rotating in and out of view as the planet orbits its host star. We further modelled the atmosphere with 3D Global Circulation Models using gCMCRT, finding that these phase dependant atmospheric models result in consistent scaling parameters throughout the orbit. This demonstrates that high resolution spectroscopy, even at R=15,000 when it cannot detect the additional Doppler shifts from offset hotspots, remains sensitive to the dynamics and 3D nature of exoplanet atmospheres. This bodes well for systems that may require the use of lower resolution spectra to improve photon collection, such as small planets orbiting in the close in habitable zones of small, faint M-dwarfs.