Phase Curves of WASP-33b and HD 149026b and a New Correlation between Phase Curve Offset and Irradiation Temperature

Abstract

We present new 3.6 and 4.5μmSpitzerphase curves for the highly irradiated hot Jupiter WASP-33b and theunusually dense Saturn-mass planet HD 149026b. As part of this analysis, we develop a new variant of pixel-leveldecorrelation that is effective at removing intrapixel sensitivity variations for long observations(>10 hr)where theposition of the star can vary by a significant fraction of a pixel. Using this algorithm, we measure eclipse depths,phase amplitudes, and phase offsets for both planets at 3.6 and 4.5μm. We use a simple toy model to show thatWASP-33b’s phase offset, albedo, and heat recirculation efficiency are largely similar to those of other hot Jupitersdespite its very high irradiation. On the other hand, ourfits for HD 149026b prefer a very high albedo. We alsocompare our results to predictions from general circulation models, and wefind that while neither planet matchesthe models well, the discrepancies for HD 149026b are especially large. We speculate that this may be related to itshigh bulk metallicity, which could lead to enhanced atmospheric opacities and the formation of reflective cloudlayers in localized regions of the atmosphere. We then place these two planets in a broader context by exploringrelationships between the temperatures, albedos, heat transport efficiencies, and phase offsets of all planets withpublished thermal phase curves. Wefind a striking relationship between phase offset and irradiation temperature:the former drops with increasing temperature until around 3400 K and rises thereafter. Although some aspects ofthis trend are mirrored in the circulation models, there are notable differences that provide important clues forfuture modeling efforts.