K巨星天龙座γ的径向速度变率：伪装成行星的恒星变率
The Radial Velocity Variability of the K-giant Gamma Draconis: Stellar Variability Masquerading as a Planet
摘要：
我们提出了从2003年到2017年对Gamma Dra进行的精确恒星径向速度测量。2003 - 2011年的数据显示出连贯的、长寿命的变化，周期为702 d。这些变化与行星伴星的存在是一致的，它的轨道特性是在演化恒星周围发现的典型巨行星。对依巴谷测光、Ca II S指数测量和这段时间内的谱线形状测量的分析表明，行星的径向速度没有变化，这似乎“证实”了行星的存在。然而，2011 - 2017年的径向速度测量似乎反驳了这一点。从2011年到2013年，径向速度变化几乎消失，只在2014年恢复，但有明显的相移。总径向速度的变化与~ 10.6年时间尺度上的振幅变化一致，也与666 d和801 d两个周期之间的拍频效应一致。这两个信号似乎都不太可能来自一个双行星系统。一个简单的动力学分析表明，只有1- 2%的机会，这两颗行星是稳定的。相反，我们认为，这种多周期性的行为可能代表了一种新形式的恒星变化，可能与振荡对流模式。如果这种内在的恒星变化在K巨星周围很常见，并归因于行星伴星，那么这些恒星中行星的出现率可能比想象的要低得多。
英文摘要：
We present precise stellar radial velocity measurements of Gamma Dra taken from 2003 to 2017. The data from 2003 to 2011 show coherent, long-lived variations with a period of 702 d. These variations are consistent with the presence of a planetary companion having m sin i = 10.7 M_Jup whose orbital properties are typical for giant planets found around evolved stars. An analysis of the Hipparcos photometry, Ca II S-index measurements, and measurements of the spectral line shapes during this time show no variations with the radial velocity of the planet which seems to "confirm"' the presence of the planet. However, radial velocity measurements taken 2011 -- 2017 seem to refute this. From 2011 to 2013 the radial velocity variations virtually disappear only to return in 2014, but with a noticeable phase shift. The total radial velocity variations are consistent either with amplitude variations on timescales of ~ 10.6 yr, or the beating effect between two periods of 666 d and 801 d. It seems unlikely that both these signals stem from a two-planet system. A simple dynamical analysis indicates that there is only a 1-2 % chance that the two-planet is stable. Rather, we suggest that this multi-periodic behavior may represent a new form of stellar variability, possibly related to oscillatory convective modes. If such intrinsic stellar variability is common around K giant stars and is attributed to planetary companions, then the planet occurrence rate among these stars may be significantly lower than thought.
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