2020ApJ...898L..31N

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_p of 226.0_–2.3^+2.1 km s^–1 and v_sys 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.