退休的A星及其伴星：围绕三个中等质量亚巨星的系外行星
Retired A Stars and Their Companions: Exoplanets Orbiting Three Intermediate-Mass Subgiants
摘要：
我们报告了来自利克和凯克天文台的三颗中等质量亚巨星的精确多普勒测量结果。所有三颗恒星的径向速度都与开普勒轨道上的行星质量同伴一致。我们发现了一颗在HD 192699周围351.5天轨道上的最小质量为2.5 Mjup的行星，一颗在HD 210702周围341.1天轨道上的最小质量为2.0 Mjup的行星，以及一颗在HD 175541周围297.3天轨道上的最小质量为0.61 Mjup的行星。根据演化模型估计的恒星质量表明，所有这些恒星都是A型矮星，质量从1.65到1.85 Msun不等。这三颗长周期行星在其恒星的主序星阶段是无法被探测到的，因为早期型矮星的自转速度和恒星抖动都很大。现在有9颗“退休”（进化）的A型恒星（M星> 1.6 Msun），拥有已知的行星。所有9颗行星的轨道距离为\geq 0.78 AU，这与低质量恒星周围行星的半长轴分布明显不同。我们研究了观测到的缺乏近距离行星的可能性，这是由于它们被膨胀的宿主恒星吞没，但我们发现这种解释是不够的，因为K巨星（R星< 32 Rsun = 0.15 AU）和亚巨星（R星< 7 Rsun = 0.03 AU）的恒星半径相对较小。相反，我们得出的结论是，行星周围的中等质量的恒星优先居住在~0.8 AU，这可能是不同的形成和迁移历史的行星周围的A型恒星的反映。
英文摘要：
We report precision Doppler measurements of three intermediate-mass subgiants from Lick and Keck Observatories. All three stars show variability in their radial velocities consistent with planet-mass companions in Keplerian orbits. We find a planet with a minimum mass of 2.5 Mjup in a 351.5 day orbit around HD 192699, a planet with a minimum mass of 2.0 Mjup in a 341.1 day orbit around HD 210702, and a planet with a minimum mass of 0.61 Mjup in a 297.3 day orbit around HD 175541. Stellar mass estimates from evolutionary models indicate that all of these stars were formerly A-type dwarfs with masses ranging from 1.65 to 1.85 Msun. These three long-period planets would not have been detectable during their stars' main-sequence phases due to the large rotational velocities and stellar jitter exhibited by early-type dwarfs. There are now 9 "retired" (evolved) A-type stars (Mstar > 1.6 Msun) with known planets. All 9 planets orbit at distances a \geq 0.78 AU, which is significantly different than the semimajor axis distribution of planets around lower-mass stars. We examine the possibility that the observed lack of close-in planets is due to engulfment by their expanding host stars, but we find that this explanation is inadequate given the relatively small stellar radii of K giants (Rstar < 32 Rsun = 0.15 AU) and subgiants (Rstar < 7 Rsun = 0.03 AU). Instead, we conclude that planets around intermediate-mass stars reside preferentially beyond ~0.8 AU, which may be a reflection of different formation and migration histories of planets around A-type stars.
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