Improving MCMC Convergence for Joint Astrometry and Radial Velocity Orbit-fits Through Reparameterization

Abstract

The exoplanet orbit-fitting software package orbitize! was initially designed to fit the orbits of directly imaged planets with relative astrometric measurements using a Markov Chain Monte Carlo (MCMC) algorithm. Since the publication of orbitize! v1.0, the ability to jointly fit radial velocities and astrometry has been incorporated. We first implemented a Basis class into orbitize! that enables users to add and fit in various orbit parameterizations. We then introduced a radial velocity-focused parameterization of the Keplerian orbital elements for the joint radial velocity and astrometry fits. We compared MCMC convergence speeds of the new radial velocity-focused basis to the original orbitize! standard basis for the system HD 190771, which has full orbital coverage in radial velocity data. We found a 16% faster convergence in time with the radial velocity-focused basis. We encourage users to consider using this basis when doing joint radial velocity and astrometry fits.