Detection of Fe i Emission in the Dayside Spectrum of WASP-33b

We analyze the high-resolution emission spectrum of WASP-33b taken using the High Dispersion Spectrograph (R ≈ 165,000) on the 8.2 m Subaru telescope. The data cover λ ≈ 6170–8817 Å, divided over 30 spectral orders. The telluric and stellar lines are removed using a de-trending algorithm, SysRem, before cross-correlating with planetary spectral templates. We calculate the templates assuming a 1D plane-parallel hydrostatic atmosphere including continuum opacity of bound–free H− and Rayleigh scattering by H2 with a range of constant abundances of Fe i. Using a likelihood-mapping analysis, we detect an Fe i emission signature at 6.4σ located at ${K}_{{\rm{p}}}$ of 226.0 ${}_{-2.3}^{+2.1}$ km s−1 and vsys of −3.2 ${}_{-1.8}^{+2.1}$ km s−1—consistent with the planet's expected velocity in the literature. We also confirm the existence of a thermal inversion in the dayside of the planet, which is very likely to be caused by the presence of Fe 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.