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We report on Fe I in the day-side atmosphere of the ultra-hot JupiterWASP-33b, providing evidence for a thermal inversion in the presence of anatomic species. We also introduce a new way to constrain the planet'sbrightness variation throughout its orbit, including its day-night contrast andpeak phase offset, using high-resolution Doppler spectroscopy alone. We do soby analyzing high-resolution optical spectra of six arcs of the planet's phasecurve, using ESPaDOnS on the Canada-France-Hawaii telescope and HDS on theSubaru telescope. By employing a likelihood mapping technique, we explore themarginalized distributions of parameterized atmospheric models, and detect Fe Iemission at high significance ($>10.4\sigma$) in our combined data sets,located at $K_{\rm p}=222.1\pm0.4$ km/s and $v_{\rm sys}=-6.5\pm0.3$ km/s. Ourvalues agree with previous reports. By accounting for WASP-33b's brightnessvariation, we find evidence that its night-side flux is $<10\%$ of the day-sideflux and the emission peak is shifted westward of the substellar point,assuming the spectrum is dominated by Fe I. Our ESPaDOnS data, which coverphases before and after the secondary eclipse more evenly, weakly constrain thephase offset to $+22\pm12$ degrees. We caution that the derivedvolume-mixing-ratio depends on our choice of temperature-pressure profile, butnote it does not significantly influence our constraints on day-night contrastor phase offset. Finally, we use simulations to illustrate how observationswith increased phase coverage and higher signal-to-noise ratios can improvethese constraints, showcasing the expanding capabilities of high-resolutionDoppler spectroscopy.

Day-side Fe I Emission, Day-Night Brightness Contrast and Phase Offset of the Exoplanet WASP-33b