EXOFAST: A Fast Exoplanetary Fitting Suite in IDL

TLDR

EXOFAST is a fast, robust IDL suite designed to fit exoplanet transits and radial‑velocity data simultaneously or separately, providing uncertainty and covariance estimates via a differential‑evolution Markov chain Monte Carlo method. The suite implements a differential‑evolution MCMC algorithm that integrates transit and RV data, offers an online interface for basic fitting, and can be extended to include additional effects or other data sets. On the HAT‑P‑3b discovery data, EXOFAST delivers self‑consistent stellar and orbital parameters, achieves over a 100‑fold speedup in transit modeling, and highlights pitfalls in eccentricity parameterization and random‑number generation that can bias MCMC results.

Abstract

We present EXOFAST, a fast, robust suite of routines written in IDL that is designed to fit exoplanetary transits and radial velocity variations simultaneously or separately and characterize the parameter uncertainties and covariances with a differential evolution Markov chain Monte Carlo method. We describe how our code incorporates both data sets to derive simultaneously stellar parameters along with the transit and RV parameters, resulting in more self-consistent results on an example fit of the discovery data of HAT-P-3b that is well-mixed in under 5 minutes on a standard desktop computer. We describe in detail how our code works and outline ways in which the code can be extended to include additional effects or generalized for the characterization of other data sets—including non-planetary data sets. We discuss the pros and cons of several common ways to parameterize eccentricity, highlight a subtle mistake in the implementation of MCMC that could bias the inferred eccentricity of intrinsically circular orbits to significantly non-zero results, discuss a problem with IDL's built-in random number generator in its application to large MCMC fits, and derive a method to analytically fit the linear and quadratic limb darkening coefficients of a planetary transit. Finally, we explain how we achieved improved accuracy and over a factor of 100 improvement in the execution time of the transit model calculation. Our entire source code, along with an easy-to-use online interface for several basic features of our transit and radial velocity fitting, are available online at http://astroutils.astronomy.ohio-state.edu/exofast.