We present high-resolution dayside thermal emission observations of the exoplanet KELT-20b/MASCARA-2b using the MAROON-X spectrograph. Applying the cross-correlation method with both empirical and theoretical masks and a retrieval analysis, we confirm previous detections of Fe\,\textsc{ i} emission lines and we detect Ni\,\textsc{ i} for the first time in the planet (at 4.7$\sigma$ confidence). We do not see evidence for additional species in the MAROON-X data, including notably predicted thermal inversion agents TiO and VO, their atomic constituents Ti\,\textsc{ i} and V\,\textsc{ i}, and previously claimed species Fe\,\textsc{ ii} and Cr\,\textsc{ i}. We also perform a joint retrieval with existing \textit{ Hubble Space Telescope}/WFC3 spectroscopy and \textit{ Spitzer}/IRAC photometry. This allows us to place bounded constraints on the abundances of Fe\,\textsc{ i}, H$_2$O, and CO, and to place a stringent upper limit on the TiO abundance. The results are consistent with KELT-20b having a solar to slightly super-solar composition atmosphere in terms of the bulk metal enrichment, and the carbon-to-oxygen and iron-to-oxygen ratios. However, the TiO volume mixing ratio upper limit (10$^{ -7.6}$ at 99\% confidence) is inconsistent with this picture, which, along with the non-detection of Ti\,\textsc{ i}, points to sequestration of Ti species, possibly due to nightside condensation. The lack of TiO but the presence of a large H$_2$O emission feature in the WFC3 data is challenging to reconcile within the context of 1D self-consistent, radiative-convective models.
Categories: astro-ph.EP
