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

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, 18Del (G6III), ξAql (K0III) and HD81688 (K0III-IV), are located in the clump region on the HRdiagram with estimated masses of 2.1-2.3M_odot. 18Del b has a minimum mass of 10.3M_J and resides in a nearly circular orbit with period of 993d, which is the longest one around evolved stars. ξAqlb and HD81688b have minimum masses of 2.8 and 2.7M_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 (1.7-3.9M_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 2-3M_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.