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 (≲1 mbar level). When analyzing only the HST data, our models favor high-metallicity atmospheres, in contrast with the previous analysis by Chachan et al. We suspect that this discrepancy arises from the different choice of chemistry modeling (free constant-with-altitude versus thermochemical equilibrium) and the enhanced parameter correlations found when neglecting the Spitzer observations. The bart source code and documentation are available at https://github.com/exosports/BART.