Confirmation of Iron Emission Lines and Non-detection of Molecules on the Dayside of KELT-9b with MAROON-X

We present dayside thermal emission observations of the hottest exoplanet KELT-9b using the new MAROON-X spectrograph. We detect atomic lines in emission at 10$\sigma$ confidence using cross correlation with binary masks. The detection of emission lines confirms the presence of a thermal inversion in KELT-9b's atmosphere. We also search for TiO and other molecules, which have been invoked to explain the unusual \textit{HST}/WFC3 spectrum of the planet. We do not detect any molecules, and instead use a retrieval approach to place an upper limit on the TiO volume mixing ratio of 10$^{-8.5}$ (at 99% confidence). This upper limit is inconsistent with the models used to match the WFC3 data, which require at least an order of magnitude more TiO, thus suggesting the need for an alternate explanation of the space-based data. Our retrieval results also strongly prefer an inverted temperature profile and atomic/ion abundances largely consistent with the expectations for a solar composition gas in thermochemical equilibrium. The exception is the retrieved abundance of Fe$^+$, which is about 1-2 orders of magnitude greater than predictions. These results highlight the growing power of high-resolution spectrographs on large ground-based telescopes to characterize exoplanet atmospheres when used in combination with new retrieval techniques.