Detection of Fe\,{\sc i} Emission in the Day-side Spectrum of WASP-33b

We analyze the high-resolution emission spectrum of WASP-33b taken using the High Dispersion Spectrograph (R\,$\approx$\,165,000) on the 8.2-m Subaru telescope. The data cover $λ$\,$\approx$\,$6170$-$8817$\,Å, divided over 30 spectral orders. The telluric and stellar lines are removed using a de-trending algorithm, {\sc SysRem}, before cross-correlating with planetary spectral templates. We calculate the templates assuming a 1-D plane-parallel hydrostatic atmosphere including continuum opacity of bound-free H$^{-}$ and Rayleigh scattering by H$_{2}$ with a range of constant abundances of Fe\,{\sc i}. Using a likelihood-mapping analysis, we detect an Fe\,{\sc i} emission signature at 6.4-$σ$ located at $K_{\mathrm{p}}$ of 226.0\,$^{+2.1}_{-2.3}$\,km\,s$^{-1}$and $v_{\mathrm{sys}}$ of -3.2\,$^{+2.1}_{-1.8}$\,km\,s$^{-1}$ -- consistent with the planet's expected velocity in the literature. We also confirm the existence of a thermal inversion in the day-side of the planet which is very likely to be caused by the presence of Fe\,{\sc i} and previously-detected TiO in the atmosphere. This makes WASP-33b one of the prime targets to study the relative contributions of both species to the energy budget of an ultra-hot Jupiter.