On the 2:1 Orbital Resonance in the HD 82943 Planetary System

We present an analysis of the HD 82943 planetary system based on a radial velocity data set that combines new measurements obtained with the Keck telescope and the CORALIE measurements published in graphical form. We examine simultaneously the goodness of fit and the dynamical properties of the best-fit double-Keplerian model as a function of the poorly constrained eccentricity and argument of periapse of the outer planet's orbit. The fit with the minimum χ²_ν is dynamically unstable if the orbits are assumed to be coplanar. However, the minimum is relatively shallow, and there is a wide range of fits outside the minimum with reasonable χ²_ν. For an assumed coplanar inclination i=30^deg (sini=0.5), only good fits with both of the lowest order, eccentricity-type mean-motion resonance variables at the 2:1 commensurability, θ₁ and θ₂, librating about 0° are stable. For sini=1, there are also some good fits with only θ₁ (involving the inner planet's periapse longitude) librating that are stable for at least 10⁸ yr. The libration semiamplitudes are about 6° for θ₁ and 10° for θ₂ for the stable good fit with the smallest libration amplitudes of both θ₁ and θ₂. We do not find any good fits that are nonresonant and stable. Thus, the two planets in the HD 82943 system are almost certainly in 2:1 mean-motion resonance, with at least θ₁ librating, and the observations may even be consistent with small-amplitude librations of both θ₁ and θ₂.

Based on observations obtained at the W. M. Keck Observatory, which is operated jointly by the University of California and the California Institute of Technology.