Tau-REx II: Retrieval of emission spectra

TL;DR

Tau-REx II introduces a hybrid Bayesian retrieval method for exoplanet emission spectra, combining parametric and layer-by-layer models to improve atmospheric temperature-pressure profile estimation.

Contribution

It presents a novel two-stage TP profile retrieval approach that merges parametric and layer-by-layer methods within the Tau-REx framework for emission spectroscopy.

Findings

Successfully applied to simulated hot-Jupiter and SuperEarth atmospheres.

Demonstrated on real secondary eclipse data of HD189733b.

Improves robustness and accuracy of atmospheric retrievals.

Abstract

Tau-REx (Tau Retrieval of Exoplanets) is a novel, fully Bayesian atmospheric retrieval code custom built for extrasolar atmospheres. In Waldmann et al. (2015) the transmission spectroscopic case was introduced, here we present the emission spectroscopy spectral retrieval for the Tau-REx framework. Compared to transmission spectroscopy, the emission case is often significantly more degenerate due to the need to retrieve the full atmospheric temperature-pressure (TP) profile. This is particularly true in the case of current measurements of exoplanetary atmospheres, which are either of low signal-to-noise, low spectral resolution or both. Here we present a new way of combining two existing approaches to the modelling of the said TP profile: 1) the parametric profile, where the atmospheric TP structure is analytically approximated by a few model parameters, 2) the Layer-by-Layer approach, where individual atmospheric layers are modelled. Both these approaches have distinct advantages and disadvantages in terms of convergence properties and potential model biases. The Tau-REx hybrid model presented here is a new two-stage TP profile retrieval, which combines the robustness of the analytic solution with the accuracy of the Layer-by-Layer approach. The retrieval process is demonstrated using simulations of the hot-Jupiter WASP-76b and the hot SuperEarth 55 Cnc e, as well as on the secondary eclipse measurements of HD189733b.