A PAIR OF GIANT PLANETS AROUND THE EVOLVED INTERMEDIATE-MASS STAR HD 47366: MULTIPLE CIRCULAR ORBITS OR A MUTUALLY RETROGRADE CONFIGURATION

Abstract

ABSTRACT We report the detection of a double planetary system around the evolved intermediate-mass star HD 47366 from precise radial-velocity measurements at the Okayama Astrophysical Observatory, Xinglong Station, and Australian Astronomical Observatory. The star is a K1 giant with a mass of 1.81 ± 0.13 M ⊙ , a radius of 7.30 ± 0.33 R ⊙ , and solar metallicity. The planetary system is composed of two giant planets with minimum masses of and , orbital periods of days and days, and eccentricities of and , respectively, which are derived by a double Keplerian orbital fit to the radial-velocity data. The system adds to the population of multi-giant-planet systems with relatively small orbital separations, which are preferentially found around evolved intermediate-mass stars. Dynamical stability analysis for the system revealed, however, that the best-fit orbits are unstable in the case of a prograde configuration. The system could be stable if the planets were in 2:1 mean-motion resonance, but this is less likely, considering the observed period ratio and eccentricity. A present possible scenario for the system is that both of the planets have nearly circular orbits, namely the eccentricity of the outer planet is less than ∼0.15, which is just within 1.4 σ of the best-fit value, or the planets are in a mutually retrograde configuration with a mutual orbital inclination larger than 160°.