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A novel 2D atmospheric temperature model for hot Jupiter phase curves retrievals

Presentation #200.05 in the session Exoplanet Atmospheres: Giant Planets (Oral Presentation)

Published onOct 23, 2023
A novel 2D atmospheric temperature model for hot Jupiter phase curves retrievals

Gaseous giant planets with close-in orbits, dubbed hot Jupiters, are among the most accessible exoplanet targets for spectroscopic observations. Since the spectral appearance of hot Jupiters is determined by the opacity structure and the thermal structure of their atmospheres, we could constrain the atmospheric properties of these planets by fitting spectra generated from atmospheric models to observations in a process known as atmospheric retrievals. The accurate determination of the elemental abundance of hot Jupiters is essential to understanding planetary formation and migration; however, the uncertainties on the chemical constraints obtained from individual spectra are typically large due to the many degeneracies inherent in low-resolution disc-average spectroscopy, especially the degeneracy between thermal structure and chemical abundance. We could partially break this degeneracy by observing phase curves of hot Jupiters, which are disc-averaged spectra at multiple orbital phases. We propose a novel 2D parametric temperature scheme, which can simultaneously fit all phases of a set of phase curve observations using Bayesian parameter estimation algorithms. The scheme consists of a dayside and a nightside, where temperature is constant on isobars on the nightside and varies with cosn(longitude/e) on the dayside, n and e being free parameters. We first validate the performance of the scheme with synthetic HST/WFC3 + Spitzer/IRAC phase curves of hot Jupiter WASP-43b simulated from a general circulation model (GCM) and find that we can accurately retrieve the latitudinally-averaged thermal structure and constrain the abundance of H2O and CH4. We then apply our scheme to the observed HST/WFC3 + Spitzer/IRAC phase curves of WASP-43b and find: (1) the dayside temperature-pressure profiles do not vary strongly with longitude and are non-inverted; (2) the retrieved nightside temperatures are extremely low, suggesting significant nightside cloud coverage; (3) the water VMR is constrained to 5.6×10-5–4.0×10-4, and we retrieve an upper bound for methane at 10-6. We have created an open-source Python package NEMESISPY for spectral simulation and phase curves retrievals to implement our scheme. Our work can be applied to JWST phase curves, and we explore two additional parameterisations: (1) vertical variability of CH4 abundance, guided by chemical and dynamical modelling work; (2) parameterisation of nightside clouds, guided by recent cloudy GCM studies.

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