ULTRACAM z'-band Detection of the Secondary Eclipse of WASP-12b

TL;DR

This study reports a deeper z'-band secondary eclipse of WASP-12b, suggesting a carbon-rich atmosphere without thermal inversion, and discusses potential atmospheric variability effects.

Contribution

First z'-band measurement of WASP-12b's secondary eclipse with ULTRACAM, providing new insights into its atmospheric composition and dynamics.

Findings

Eclipse depth of 0.130% measured, deeper than previous results.

Orbit consistent with being circular, phase 0.501.

Atmosphere likely carbon-rich, no strong thermal inversion.

Abstract

We present z'-band secondary eclipse photometry of the highly irradiated hot Jupiter WASP-12b using ULTRACAM on the 4.2m William Herschel Telescope. We measure a decrease in flux of \delta = 0.130+/-0.013% during the passage of the planet behind the star, which is significantly deeper than the previous measurement at this wavelength (0.082+/-0.015%, L\'opez-Morales et al. 2010). Our secondary eclipse is best fit with a mid-eclipse phase, \phi, that is compatible with a circular orbit \phi = 0.501+/-0.002, in agreement with previous results (Croll et al. 2011). In combination with existing data, our eclipse depth measurement allows us to constrain the characteristics of the planet's atmosphere, which is consistent with a carbon-rich model, with no evidence for a strong thermal inversion. If the difference in eclipse depth reported here compared to that of L\'opez-Morales et al. (2010) is of physical origin, as opposed to due to systematics, it may be caused by temporal variability in the flux, due to atmospheric dynamics.