Two Jovian Planets around the Giant Star HD 202696: A Growing Population of Packed Massive Planetary Pairs around Massive Stars?

Abstract

We present evidence for a new two-planet system around the giant star HD 202696 (=HIP 105056, BD +26 4118). The discovery is based on public HIRES radial velocity (RV) measurements taken at Keck Observatory between 2007 July and 2014 September. We estimate a stellar mass of {1.91}_-0.14^+0.09{M}_⊙ for HD 202696, which is located close to the base of the red giant branch. A two-planet self-consistent dynamical modeling MCMC scheme of the RV data followed by a long-term stability test suggests planetary orbital periods of P _b = {517.8}_-3.9^+8.9 and P _c = {946.6}_-20.9^+20.7 days, eccentricities of e _b = {0.011}_-0.011^+0.078 and e _c = {0.028}_-0.012^+0.065, and minimum dynamical masses of m _b = {2.00}_-0.10^+0.22 and m _c = {1.86}_-0.23^+0.18 M _Jup, respectively. Our stable MCMC samples are consistent with orbital configurations predominantly in a mean period ratio of 11:6 and its close-by high-order mean-motion commensurabilities with low eccentricities. For the majority of the stable configurations, we find an aligned or anti-aligned apsidal libration (i.e., Δω librating around 0° or 180°), suggesting that the HD 202696 system is likely dominated by secular perturbations near the high-order 11:6 mean-motion resonance. The HD 202696 system is yet another Jovian-mass pair around an intermediate-mass star with a period ratio below the 2:1 mean-motion resonance. Therefore, the HD 202696 system is an important discovery that may shed light on the primordial disk-planet properties needed for giant planets to break the strong 2:1 mean-motion resonance and settle in more compact orbits.