Title: Evidence for a Dayside Thermal Inversion and High Metallicity for the Hot Jupiter WASP-18b,
Abstract: We find evidence for a strong thermal inversion in the dayside atmosphere of
the highly irradiated hot Jupiter WASP-18b (T$_{eq}=2411K$, $M=10.3M_{J}$)
based on emission spectroscopy from Hubble Space Telescope secondary eclipse
observations and Spitzer eclipse photometry. We demonstrate a lack of water
vapor in either absorption or emission at 1.4$\mu$m. However, we infer emission
at 4.5$\mu$m and absorption at 1.6$\mu$m that we attribute to CO, as well as a
non-detection of all other relevant species (e.g., TiO, VO). The most probable
atmospheric retrieval solution indicates a C/O ratio of 1 and a high
metallicity (C/H=$283^{+395}_{-138}\times$ solar). The derived composition and
T/P profile suggest that WASP-18b is the first example of both a planet with a
non-oxide driven thermal inversion and a planet with an atmospheric metallicity
inconsistent with that predicted for Jupiter-mass planets at $>2\sigma$. Future
observations are necessary to confirm the unusual planetary properties implied
by these results.