Planets around the evolved stars 24?Bo?tis and γ?Libra: A 30 d-period planet and a double giant-planet system in possible 7:3 MMR

摘要

We report the detection of planets around two evolved giant stars from radial velocity measurements at Okayama Astrophysical observatory. 24?Boo (G3?IV) has a mass of |$0.99,M_{odot }$|?, a radius of |$10.64,R_{odot }$|?, and a metallicity of [Fe/H]?=??0.77. The star hosts one planet with a minimum mass of 0.91?MJup and an orbital period of 30.35?d. The planet has one of the shortest orbital periods among those ever found around evolved stars using radial-velocity methods. The stellar radial velocities show additional periodicity with 150?d, which can probably be attributed to stellar activity. The star is one of the lowest-metallicity stars orbited by planets currently known. γ?Lib (K0?III) is also a metal-poor giant with a mass of |$1.47,M_{odot }$|?, a radius of |$11.1,R_{odot }$|?, and [Fe/H]?=??0.30. The star hosts two planets with minimum masses of 1.02?MJup and 4.58?MJup, and periods of 415?d and 964?d, respectively. The star has the second-lowest metallicity among the giant stars hosting more than two planets. Dynamical stability analysis for the γ?Lib system sets the minimum orbital inclination angle to be about 70° and suggests that the planets are in 7:3 mean-motion resonance, though the current best-fitting orbits for the radial-velocity data are not totally regular.