围绕三个中等质量G和K巨星的行星伴星：18 Del，xi Aql和HD 81688
Planetary Companions around Three Intermediate-Mass G and K Giants: 18 Del, xi Aql, and HD 81688
摘要：
本文报道冈山天文台在对G和K巨星的精密多普勒巡天中发现了3颗新的太阳系外行星。主恒星18 Del（G6 III）、xi Aql（K 0 III）和HD 81688（K 0 III-IV）在HR图上位于团簇区，质量估计为2.1- 2.3M_solar。18 Del b的最小质量为10.3 M_Jup，轨道近似为圆形，周期为993天，是迄今为止发现的最长的演化恒星轨道。xi Aql b和HD 81688 b的最小质量分别为2.8和2.7 M_Jup，轨道周期分别为137和184天，是围绕演化恒星运行的行星中最短的。在可能的中等质量（1.7-3.9 M_solar）团巨星周围发现的所有亚恒星伴星的半长轴都大于0.68 AU，这表明缺乏短周期行星。我们的数值计算表明，由于来自中心恒星的潮汐力矩，2-3 M太阳恒星周围0.5 AU（甚至1 AU，取决于金属丰度和采用的模型）以内的太阳质量行星可能被RGB尖端的中心恒星吞没。假设大多数的团状巨星都是后RGB恒星，我们无法区分短周期行星的缺乏是原始的还是由于被中心恒星吞噬。为进一步研究中等质量恒星周围行星系统的形成和演化，需要获得演化恒星的可靠质量和演化状态。
英文摘要：
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_solar. 18 Del b has a minimum mass of 10.3 M_Jup 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_Jup, 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_solar) 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_solar 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.
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