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

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-circle dot, a radius of 7.30 +/- 0.33 R-circle dot, and solar metallicity. The planetary system is composed of two giant planets with minimum masses of 1.75(-0.17)(+0.20) M-J and 1.86(-0.15)(+0.16) M-J, orbital periods of 363.3(-2.4)(+2.5) days and 684.7(-4.9)(+5.0) days, and eccentricities of 0.089(-0.060)(+0.079) and 0.278(-0.094)(+0.067), 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 similar to 0.15, which is just within 1.4 sigma of the best-fit value, or the planets are in a mutually retrograde configuration with a mutual orbital inclination larger than 160 degrees.