Planets around the evolved stars 24 Booties and $\gamma$ Libra: A 30d-period planet and a double giant-planet system in possible 7:3 MMR

Abstract

We report the detection of planets around two evolved giant stars from radial velocity measurements at Okayama Astrophysical observatory. 24 Boo (G3IV) has a mass of $0.99M_{\odot }$, a radius of $10.64R_{\odot }$, and a metallicity of $[Fe/H]=-0.77$. The star hosts one planet with a minimum mass of $0.91M_{\mathrm{Jup}}$ and an orbital period of $30.35\mathrm{d}$. The planet has one of the shortest orbital periods among those ever found around evolved stars by radial-veloocity methods. The stellar radial velocities show additional periodicity with $150\mathrm{d}$, which are probably attributed to stellar activity. The star is one of the lowest-metallicity stars orbited by planets currently known. $\gamma$ Lib (K0III) is also a metal-poor giant with a mass of $1.47M_{\odot }$, a radius of $11.1R_{\odot }$, and $[Fe/H]=-0.30$. The star hosts two planets with minimum masses of $1.02M_{\mathrm{Jup}}$ and $4.58M_{\mathrm{Jup}}$, and periods of $415\mathrm{d}$ and $964\mathrm{d}$, respectively. The star has the second lowest metallicity among the giant stars hosting more than two planets. Dynamical stability analysis for the $\gamma$ Lib system sets a minimum orbital inclination angle to be about $70^{\circ }$ and suggests that the planets are in 7:3 mean-motion resonance, though the current best-fitted orbits to the radial-velocity data are not totally regular.

29 pages, 19 figures, accepted for publication in PASJ