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

Abstract

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, $R\sim 15,000$), covering pre- and post-eclipse phases, we cross-correlate with 1D PHOENIX spectral templates to detect CO at S/N = 7.9 ($v_{sys}=0.15_{-0.65}^{+0.64}$ km/s, $K_p=229.5_{-1.0}^{+1.1}$ km/s). 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.

24 pages, 21 figures, accepted to MNRAS

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