An Open-source Bayesian Atmospheric Radiative Transfer (BART) Code. II. The TRANSIT Radiative Transfer Module and Retrieval of HAT-P-11b

Abstract

This and companion papers by Harrington et al. and Blecic et al. present the Bayesian Atmospheric Radiative Transfer (BART) code, an open-source, open-development package to characterize extrasolar planet atmospheres. BART combines a thermochemical equilibrium abundance (TEA), a radiative transfer (TRANSIT), and a Bayesian statistical (MC3) module to constrain atmospheric temperatures and molecular abundances for given spectroscopic observations. Here we describe the TRANSIT radiative transfer package, an efficient line-by-line radiative transfer C code for one-dimensional atmospheres, developed by P. Rojo and further modified by the UCF exoplanet group. This code produces transmission and hemisphere-integrated emission spectra. TRANSIT handles line-by-line opacities from HITRAN, Partridge & Schwenke (H2O), Schwenke (TiO), and Plez (VO) and collision-induced absorption from Borysow, HITRAN, and ExoMol. TRANSIT emission spectra models agree with models from C. Morley (private communication) within a few percent. We applied BART to the Spitzer and Hubble transit observations of the Neptune-sized planet HAT-P-11b. Our analysis of the combined HST and Spitzer data generally agrees with those from previous studies, finding atmospheric models with enhanced metallicity (>̰100× solar) and high-altitude clouds (