Two Jovian planets around the giant star HD202696. 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 HD202696 (= HIP105056, BD+26 4118). The discovery is based on public HIRES radial velocity measurements taken at Keck Observatory between July 2007 and September 2014. We estimate a stellar mass of 1.91${}_{-0.14}^{+0.09}{M}_{\odot }$ for HD202696, which is located close to the base of the red giant branch. A two-planet self-consistent dynamical modeling MCMC scheme of the radial velocity data followed by a long-term stability test suggests planetary orbital periods of $P_{\mathrm{b}}$ = 517.8${}_{-3.9}^{+8.9}$ days and $P_{\mathrm{c}}$ = 946.6${}_{-20.9}^{+20.7}$ days, eccentricities of $e_{\mathrm{b}}$ = 0.011${}_{-0.011}^{+0.078}$ and $e_{\mathrm{c}}$ = 0.028${}_{-0.012}^{+0.065}$ , and minimum dynamical masses of $m_{\mathrm{b}}$ = 2.00${}_{-0.10}^{+0.22}$\,$M_{\mathrm{Jup}}$ and $m_{\mathrm{c}}$ = 1.86${}_{-0.23}^{+0.18}$,$M_{\mathrm{Jup}}$, 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.\ $\Delta \omega$ librating around 0${}^{\circ }$ or 180${}^{\circ }$), suggesting that the HD202696 system is likely dominated by secular perturbations near the high-order 11:6 mean-motion resonance. The HD202696 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 HD202696 system is an important discovery, which 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.

Accepted for publication in AJ on 18th December 2018. 15 pages, 11 Figures, 4 Tables (this .v2 is copy corrected and identical with the published paper)

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