Evidence for a Dayside Thermal Inversion and High Metallicity for the Hot Jupiter WASP-18b

Kyle Sheppard, Avi M. Mandell, Patrick Tamburo, Siddarth Gandhi, Arazi Pinhas, Nikku Madhusudhan, Drake Deming
(Submitted on 28 Nov 2017)

We find evidence for a strong thermal inversion in the dayside atmosphere of the highly irradiated hot Jupiter WASP-18b (Teq=2411K, M=10.3MJ) 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 1.4μm. However, we infer emission at 4.5μm and absorption at 1.6μm 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× 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 >2σ. Future observations are necessary to confirm the unusual planetary properties implied by these results.

Comments:    7 pages, 4 figures, 1 table, published in ApJ Letters 11/29
Subjects:    Earth and Planetary Astrophysics (astro-ph.EP)
DOI:    10.3847/2041-8213/aa9ae9
Cite as:    arXiv:1711.10491 [astro-ph.EP] (or arXiv:1711.10491v1 [astro-ph.EP] for this version)
Submission history
From: Kyle Sheppard
[v1] Tue, 28 Nov 2017 19:00:01 GMT (3741kb,D)
https://arxiv.org/abs/1711.10491