We report the detection of 3 new extrasolar planets from the precise Doppler survey of G and K giants at Okayama Astrophysical Observatory .
The host stars , namely , 18 Del ( G6 III ) , \xi Aql ( K0 III ) and HD 81688 ( K0 III–IV ) , are located at the clump region on the HR diagram with estimated masses of 2.1 - 2.3 M _ { \odot } .
18 Del b has a minimum mass of 10.3 M _ { J } and resides in a nearly circular orbit with period of 993 days , which is the longest one ever discovered around evolved stars . \xi Aql b and HD 81688 b have minimum masses of 2.8 and 2.7 M _ { J } , and reside in nearly circular orbits with periods of 137 and 184 days , respectively , which are the shortest ones among planets around evolved stars .
All of the substellar companions ever discovered around possible intermediate-mass ( 1.7 - 3.9 M _ { \odot } ) clump giants have semimajor axes larger than 0.68 AU , suggesting the 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 2–3 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 .
Deriving reliable mass and evolutionary status for evolved stars is highly required for further investigation of formation and evolution of planetary systems around intermediate-mass stars .