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 µm. However, we infer emission at 4.5 µm and absorption at 1.6 µm 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. Subject headings: planets and satellites: atmospheres -planets and satellites: composition -planets and satellites: gaseous planets -planets and satellites: individual(WASP-18b)

Suppressed Far-UV Stellar Activity and Low Planetary Mass Loss in the WASP-18 System*

“…Mass loss is mostly driven by Jeans escape, hence it is not energy limited. The possible metal rich nature of the planetary atmosphere (Sheppard et al 2017) is therefore likely to be primordial.…”

“…high metallicity derived by Sheppard et al (2017) is most likely to be primordial and not caused by selective erosion (i.e. lighter elements are more likely to escape compared to heavier ones), even considering that the star may have been more active in the past.…”

“…We calculate the basic atmospheric structure to set the lower boundary conditions of our escape model by assuming an isothermal lower atmosphere with a dayside temperature of 3150 K 9 (Nymeyer et al 2011;Sheppard et al 2017). This is a rather extreme choice that leads to significant dissociation of H 2 above the 0.1 bar level.…”

“…Because of the close proximity (about 0.02 au; Southworth 2012) to a rather hot star, the planet has a very high equilibrium temperature of 2413±44 K (Southworth 2012). Nymeyer et al (2011) and Sheppard et al (2017) measured the planet's brightness temperature from Spitzer secondary eclipse observations, obtaining a value of about 3100 K. From the analysis of HST secondary eclipse observations, Sheppard et al (2017) further concluded that WASP-18 hosts a metal rich atmosphere (about 200× solar).…”

Strong biases in retrieved atmospheric composition caused by day–night chemical heterogeneities

“…Currently, only a few UHJs have been discovered and studied using both transit and eclipse spectroscopy (Wright et al 2012;Haynes et al 2015;Sheppard et al 2017;Evans et al 2017;). However, the analysis of UHJs is not simple, due to their complex chemical composition and dynamics.…”