Planetary companions around three intermediate-mass G and K Giants: 18 Delphini, ξ aquilae, and HD 81688

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), ξ Aql (K0 III) and HD 81688 (K0 III-IV), are located in the clump region on the HR diagram with estimated masses of 2.1-2.3M⊙. 18 Del b has a minimum mass of 10.3MJ and resides in a nearly circular orbit with period of 993 d, which is the longest one around evolved stars. ξ Aql b and HD 81688 b 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⊙) clump giants have semimajor axes larger than 0.68 AU, suggesting a lack of short-period planets. Our numerical calculations suggest that Jupiter-mass planets within about 0.5 AU (even up to 1 AU, depending on the metallicity and adopted models) around 2-3M⊙ 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. © 2008. Astronomical Society of Japan.