LDTK: Limb Darkening Toolkit

We present a PYTHON package LDTK that automates the calculation of custom stellar limb-darkening (LD) profiles and model-specific limb-darkening coefficients using the library of PHOENIX-generated specific intensity spectra by Husser et al. The aim of the package is to facilitate analyses requiring custom generated LD profiles, such as the studies of exoplanet transits - especially transmission spectroscopy, where the transit modelling is carried out for custom narrow passbands - eclipsing binaries, interferometry, and microlensing events. First, LDTK can be used to compute custom LD profiles with uncertainties propagated from the uncertainties in the stellar parameter estimates. Secondly, LDTK can be used to estimate the LD-model-specific coefficients with uncertainties for the most common LD models. Thirdly, LDTK can be directly integrated into the log posterior computation of any pre-existing-modelling code with minimal modifications. The last approach can be used to constrain the LD model parameter space directly by the LD profile, allowing for the marginalization over the LD parameter space without the need to approximate the constraint from the LD profile using a prior.