2022AJ....163..248H

We report on Fe I in the dayside atmosphere of the ultra-hot-Jupiter WASP-33b, providing evidence for a thermal inversion in the presence of an atomic species. We also introduce a new way to constrain the planet's brightness variation throughout its orbit, including its day-night contrast and peak phase offset, using high-resolution Doppler spectroscopy alone. We do so by analyzing high-resolution optical spectra of six arcs of the planet's phase curve, using Echelle SpectroPolarimetric Device for the Observation of Stars (ESPaDOnS) on the Canada-France-Hawaii telescope and High Dispersion Spectrograph on the Subaru telescope. By employing a likelihood mapping technique, we explore the marginalized distributions of parameterized atmospheric models, and detect Fe I emission at high significance (>10.4σ) in our combined data sets, located at K_p = 222.1 ± 0.4 km s^–1 and v_sys = -6.5 ± 0.3 km s^–1. Our values agree with previous reports. By accounting for WASP-33b's brightness variation, we find evidence that its nightside flux is <10% of the dayside flux and the emission peak is shifted westward of the substellar point, assuming the spectrum is dominated by Fe I. Our ESPaDOnS data, which cover phases before and after the secondary eclipse more evenly, weakly constrain the phase offset to +22 ± 12 degrees. We caution that the derived volume-mixing ratio depends on our choice of temperature-pressure profile, but note it does not significantly influence our constraints on day-night contrast or phase offset. Finally, we use simulations to illustrate how observations with increased phase coverage and higher signal-to-noise ratios can improve these constraints, showcasing the expanding capabilities of high-resolution Doppler spectroscopy.