[论文解读] Planets around evolved intermediate-mass stars in open clusters II. Are there really planets around IC4651No9122, NGC2423No3 and NGC4349No127?

E. Delgado Mena, C. Lovis+11 more|arXiv (Cornell University)|Jul 25, 2018

Stellar, planetary, and galactic studies参考文献 85被引用 11

一句话总结

本研究利用来自HARPS和UVES的15年以上高精度径向速度（RV）数据，重新评估了在疏散星团IC4651 No. 9122、NGC2423 No. 3和NGC4349 No. 127中围绕演化中间质量恒星的三个候选亚恒星伴星的行星性质。研究发现，先前归因于行星的RV信号很可能是由恒星活动、脉动或轮廓线变化（特别是FWHM和Hα指数调制）引起的，警示长期监测对于区分演化恒星中行星信号与恒星引起的变异性至关重要。

ABSTRACT

(shorter version)The aim of this work is to search for planets around intermediate-mass stars in open clusters by using RV data obtained with HARPS from an extensive survey with more than 15 years of observations for a sample of 142 giant stars in 17 open clusters. We present the discovery of a periodic RV signal compatible with the presence of a planet candidate in the 1.15 Gyr open cluster IC4651 orbiting the 2.06 M$_\odot$ star No. 9122. If confirmed, the planet candidate would have a minimum mass of 7.2 M$_{J}$ and a period of 747 days. However, we also find that the FWHM of the CCF varies with a period close to the RV, casting doubts on the planetary nature of the signal. We also provide refined parameters for the previously discovered planet around NGC2423 No. 3 but show evidence that the BIS of the CCF is correlated with the RV during some of the observing periods. This fact advises us that this might not be a real planet and that the RV variations could be caused by stellar activity and/or pulsations. Finally, we show that the previously reported signal by a brown dwarf around NGC4349 No. 127 is presumably produced by stellar activity modulation. The long-term monitoring of several red giants in open clusters has allowed us to find periodic RV variations in several stars. However, we also show that the follow-up of this kind of stars should last more than one orbital period to detect long-term signals of stellar origin. This work warns that although it is possible to detect planets around red giants, large-amplitude, long-period RV modulations do exist in such stars that can mimic the presence of an orbiting planetary body. Therefore, we need to better understand how such RV modulations behave as stars evolve along the RGB and perform a detailed study of all the possible stellar-induced signals (e.g. spots, pulsations, granulation) to comprehend the origin of RV variations.

研究动机与目标

• 利用长期径向速度监测，重新评估先前报告的三颗演化中间质量恒星在疏散星团中亚恒星伴星的行星性质。
• 调查红巨星中周期性RV变化是否由行星伴星引起，或源于恒星活动与脉动。
• 通过分析FWHM、BIS和Hα指数等辅助指标，评估径向速度信号的可靠性。
• 确定长期监测是否能够区分演化恒星中行星轨道与恒星引起的RV调制。
• 通过识别和表征源自恒星的信号，改进围绕演化恒星的系外行星候选体的验证标准。

提出的方法

• 利用拉西拉天文台3.6米望远镜上的HARPS光谱仪，在15年以上的时间内获取高精度径向速度（RV）测量数据。
• 对表现出显著周期性RV信号的恒星进行开普勒轨道拟合，以评估行星伴星候选体。
• 监测包括交叉相关函数（CCF）的全宽半高（FWHM）、bisector跨度（BIS）和Hα指数在内的恒星活动指标，以检测其与RV变化的相关性。
• 对RV和活动指标进行周期图分析，识别共同周期性并评估误报概率（FAP）。
• 比较RV变化与FWHM、BIS和Hα指数的相位和振幅，以判断恒星活动是否可能模仿行星信号。
• 利用长期基线数据评估相关性随时间的稳定性，并检测长期的恒星调制模式。

实验结果

研究问题

• RQ1 IC4651 No. 9122的747天RV信号是否真正指示一颗7.2 M_J行星，还是由恒星活动引起？
• RQ2 NGC2423 No. 3中RV与BIS变化之间的相关性是否表明存在行星伴星，还是信号源于恒星本身？
• RQ3 NGC4349 No. 127中的RV调制是否可由恒星活动或脉动解释，而非棕矮星伴星？
• RQ4 FWHM和Hα指数变化与演化恒星中RV信号的相关性程度如何？它们能否用于验证或排除行星候选体？
• RQ5 长期恒星振荡与活动周期如何影响红巨星中径向速度系外行星探测的可靠性？

主要发现

• IC4651 No. 9122周围7.2 M_J的行星候选体（747天轨道周期）很可能并不存在，因为CCF的FWHM和Hα指数显示出与RV信号周期相近的周期性变化，表明其起源于恒星。
• NGC2423 No. 3中的RV变化在整个数据集中与CCF的BIS存在强烈相关性，这削弱了行星解释，尽管该相关性在时间上并不稳定。
• NGC4349 No. 127中的RV信号最合理的解释是恒星活动调制，因为FWHM和Hα指数在与RV信号相同的周期上表现出强烈的周期性变化，且显著性达到1%的FAP水平。
• 本研究证实，演化恒星中长周期、大振幅的RV调制可能模仿行星信号，尤其当由脉动或活动区引起时。
• CCF的FWHM是识别源自恒星的RV变化的关键诊断工具，应在验证围绕演化恒星的行星候选体时常规使用。
• 研究结果强调，必须进行长期监测（超过一个轨道周期）才能在红巨星中区分行星信号与恒星活动及脉动效应。