Planetary Companions around Three Intermediate-Mass G and K Giants: 18 Delphini, ξ Aquilae, and HD 81688

Abstract

Abstract We report on the detection of 3 new extrasolar planets from a precise Doppler survey of G and K giants at Okayama Astrophysical Observatory. The host stars, 18 Del (G6 III), $\xi $Aql (K0 III) and HD81688 (K0 III-IV), are located in the clump region on the HR diagram with estimated masses of 2.1-2.3$M_{\odot}$. 18Del b has a minimum mass of $10.3 M_{\rm J}$ and resides in a nearly circular orbit with period of 993d, which is the longest one around evolved stars. $\xi $ Aql b and HD81688 b have minimum masses of 2.8 and 2.7$M_{\rm J}$, and reside in nearly circular orbits with periods of 137 and 184d, respectively, which are the shortest ones around evolved stars. All of the substellar companions ever discovered around intermediate-mass (1.7-3.9$M_{\odot}$) clump giants have semimajor axes larger than 0.68AU, suggesting a lack of short-period planets. Our numerical calculations suggest that Jupiter-mass planets within about 0.5AU (even up to 1AU, depending on the metallicity and adopted models) around 2-3$M_{\odot}$ stars could be engulfed by the central stars at the tip of RGB due to tidal torque from the central stars. Assuming that most of the clump giants are post-RGB stars, we can not distinguish whether the lack of short-period planets is primordial, or due to engulfment by central stars.