DAY-SIDE z′-BAND EMISSION AND ECCENTRICITY OF WASP-12b

We report the detection of the eclipse of the very hot Jupiter WASP-12b via z'-band time-series photometry obtained with the 3.5 m Astrophysical Research Consortium telescope at Apache Point Observatory. We measure a decrease in flux of 0.082% +/- 0.015% during the passage of the planet behind the star. That planetary flux is equally well reproduced by atmospheric models with and without extra absorbers, and blackbody models with f >= 0.585 +/- 0.080. It is therefore necessary to measure the planet at other wavelengths to further constrain its atmospheric properties. The eclipse appears centered at phase phi = 0.5100(-0.0061)(+ 0.0072), consistent with an orbital eccentricity of vertical bar e cos omega| = 0.016(-0.009)(+ 0.011) (see note at the end of Section 4). If the orbit of the planet is indeed eccentric, the large radius of WASP-12b can be explained by tidal heating.