2.5-D retrieval of atmospheric properties from exoplanet phase curves: Application to WASP-43b observations

Patrick G. J. Irwin; Vivien Parmentier; Jake Taylor; Jo Barstow; Suzanne Aigrain; Elspeth K. H. Lee; Ryan Garland

doi:10.1093/mnras/staa238

paper

2.5-D retrieval of atmospheric properties from exoplanet phase curves: Application to WASP-43b observations

arXiv:1909.03233 · doi:10.1093/mnras/staa238

Abstract

We present a novel retrieval technique that attempts to model phase curve observations of exoplanets more realistically and reliably, which we call the 2.5-dimension (2.5-D) approach. In our 2.5-D approach we retrieve the vertical temperature profile and mean gaseous abundance of a planet at all longitudes and latitudes \textbf{simultaneously}, assuming that the temperature or composition, $x$ , at a particular longitude and latitude $(Λ, Φ)$ is given by $x (Λ, Φ) = x ˉ + (x (Λ, 0) - x ˉ) cos^{n} Φ$ , where $x ˉ$ is the mean of the morning and evening terminator values of $x (Λ, 0)$ , and $n$ is an assumed coefficient. We compare our new 2.5-D scheme with the more traditional 1-D approach, which assumes the same temperature profile and gaseous abundances at all points on the visible disc of a planet for each individual phase observation, using a set of synthetic phase curves generated from a GCM-based simulation. We find that our 2.5-D model fits these data more realistically than the 1-D approach, confining the hotter regions of the planet more closely to the dayside. We then apply both models to WASP-43b phase curve observations of HST/WFC3 and Spitzer/IRAC. We find that the dayside of WASP-43b is apparently much hotter than the nightside and show that this could be explained by the presence of a thick cloud on the nightside with a cloud top at pressure $< 0.2$ bar. We further show that while the mole fraction of water vapour is reasonably well constrained to $(1 - 10) \times 1 0^{- 4}$ , the abundance of CO is very difficult to constrain with these data since it is degenerate with temperature and prone to possible systematic radiometric differences between the HST/WFC3 and Spitzer/IRAC observations. Hence, it is difficult to reliably constrain C/O.

arXiv commentSubmitted to M.N.R.A.S. Revised Submission 9th December 2019

2.5-D retrieval of atmospheric properties from exoplanet phase curves: Application to WASP-43b observations

Abstract

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