Secondary eclipses of WASP-18b -- Near Infrared observations with the Anglo Australian Telescope, the Magellan Clay Telescope and the LCOGT network

Abstract

We present new eclipse observations for one of the hottest "hot Jupiters" WASP-18b, for which previously published data from HST WFC3 and Spitzer have led to radically conflicting conclusions about the composition of this planet's atmosphere. We measure eclipse depths of \(0.15\pm0.02\%\) at \(Ks\) and \(0.07\pm0.01\%\) at \(z'\) bands. Using the VSTAR line-by-line radiative transfer code and both these new observations with previously published data, we derive a new model of the planetary atmosphere. We have varied both the metallicity and C/O ratio in our modelling, and find no need for the extreme metallicity suggested by Sheppard et al.(2017). Our best fitting models slightly underestimate the emission at \(z'\) band and overestimate the observed flux at \(Ks\)-band. To explain these discrepancies, we examine the impact on the planetary emission spectrum of the presence of several types of hazes which could form on the night-side of the planet. Our \(Ks\) band eclipse flux measurement is lower than expected from clear atmosphere models and this could be explained by a haze particles larger than 0.2 \(\mu\)m with the optical properties of Al\(_{2}\)O\(_{3}\), CaTiO\(_{3}\) or MgSiO\(_{3}\). We find that \(z'\) band measurements are important for understanding the contribution of photochemical hazes with particles smaller than 0.1 \(\mu\)m at the top of the atmosphere.