2023MNRAS.522.2145V

We report the first detection of CO emission at high spectral resolution in the day-side infrared thermal spectrum of an exoplanet. These emission lines, found in the atmosphere of the transiting ultra hot Jupiter (UHJ) WASP-33 b, provide unambiguous evidence of its thermal inversion. Using spectra from the MMT Exoplanet Atmosphere Survey (MEASURE, R ∼ 15 000), covering pre- and post-eclipse phases, we cross-correlate with 1D PHOENIX spectral templates to detect CO at S/N = 7.9 ($v_{\rm {sys}}=0.15^{+0.64}_{-0.65}$ km s^–1, $K_{\rm {p}}=229.5^{+1.1}_{-1.0}$ km s^–1). Moreover, using cross-correlation-to-log-likelihood mapping, we find that the scaling parameter which controls the spectral line contrast changes with phase. We thus use the general circulation model SPARC/MITgcm post-processed by the 3D gCMCRT radiative transfer code to interpret this variation, finding it consistent with an eastward-shifted hot spot. Pre-eclipse, when the hot spot faces Earth, the thermal profiles are shallower leading to smaller line contrast despite greater overall flux. Post-eclipse, the western part of the day-side faces Earth and has much steeper thermal profiles, leading to larger line contrast despite less overall flux. This demonstrates that within the log-likelihood framework, even relatively moderate resolution spectra can be used to understand the 3D nature of close-in exoplanets, and that resolution can be traded for photon-collecting power when the induced Doppler-shift is sufficiently large. We highlight CO as a good probe of UHJ thermal structure and dynamics that does not suffer from stellar activity, unlike species that are also present in the host star e.g. iron lines.