${\mathcal{T}}$-REx. II. RETRIEVAL OF EMISSION SPECTRA

Abstract

Tau-REx (Tau Retrieval of Exoplanets) is a novel, fully Bayesian atmospheric\nretrieval code custom built for extrasolar atmospheres. In Waldmann et al.\n(2015) the transmission spectroscopic case was introduced, here we present the\nemission spectroscopy spectral retrieval for the Tau-REx framework. Compared to\ntransmission spectroscopy, the emission case is often significantly more\ndegenerate due to the need to retrieve the full atmospheric\ntemperature-pressure (TP) profile. This is particularly true in the case of\ncurrent measurements of exoplanetary atmospheres, which are either of low\nsignal-to-noise, low spectral resolution or both. Here we present a new way of\ncombining two existing approaches to the modelling of the said TP profile: 1)\nthe parametric profile, where the atmospheric TP structure is analytically\napproximated by a few model parameters, 2) the Layer-by-Layer approach, where\nindividual atmospheric layers are modelled. Both these approaches have distinct\nadvantages and disadvantages in terms of convergence properties and potential\nmodel biases. The Tau-REx hybrid model presented here is a new two-stage TP\nprofile retrieval, which combines the robustness of the analytic solution with\nthe accuracy of the Layer-by-Layer approach. The retrieval process is\ndemonstrated using simulations of the hot-Jupiter WASP-76b and the hot\nSuperEarth 55 Cnc e, as well as on the secondary eclipse measurements of\nHD189733b.\n