A temperature inversion in WASP-33b? Large Binocular Telescope occultation data confirm significant thermal flux at short wavelengths

TL;DR

This study presents optical and near-infrared occultation observations of WASP-33b, revealing a high brightness temperature and suggesting the planet's emission can be modeled as a simple blackbody, with no definitive evidence for a temperature inversion.

Contribution

First simultaneous optical and near-infrared occultation measurements of WASP-33b, modeling pulsations to accurately determine eclipse depth and planetary temperature.

Findings

Eclipse depth of 1.03 ± 0.34 ppt measured.

Brightness temperature of 3398 ± 302 K for WASP-33b.

Data consistent with blackbody emission, no need for complex inversion models.

Abstract

We observed a secondary eclipse of WASP-33b quasi-simultaneously in the optical (~0.55 {\mu}m) and the near-infrared (~1.05 {\mu}m) using the 2x8.4 m Large Binocular Telescope. WASP-33 is a {\delta} Scuti star pulsating with periods comparable to the eclipse duration, making the determination of the eclipse depth challenging. We use previously determined oscillation frequencies to model and remove the pulsation signal from the light curves, isolating the secondary eclipse. The determined eclipse depth is dF = 1.03 +/- 0.34 parts per thousand, corresponding to a brightness temperature of Tb = 3398 +/- 302 K. Combining previously published data with our new measurement we find the equilibrium temperature of WASP-33b to be Tb = 3358 +/- 165 K. We compare all existing eclipse data to a blackbody spectrum, to a carbon-rich non-inverted model and to a solar composition model with an inverted temperature structure. We find that current available data on WASP-33b's atmosphere can be best represented by a simple blackbody emission, without the need for more sophisticated atmospheric models with temperature inversions. Although our data cannot rule out models with or without a temperature inversion, they do confirm a high brightness temperature for the planet at short wavelengths. WASP-33b is one of the hottest exoplanets known till date, and its equilibrium temperature is consistent with rapid reradiation of the absorbed stellar light and a low albedo.