[论文解读] Transition from spot to faculae domination -- An alternate explanation for the dearth of intermediate extit{Kepler} rotation periods

Timo Reinhold, Keaton J. Bell+3 more|arXiv (Cornell University)|Oct 26, 2018

Stellar, planetary, and galactic studies参考文献 61被引用 17

一句话总结

本研究提出，15–25天的中等周期Kepler旋转周期稀少现象并非源于Vaughan-Preston间隙，而是由于在约8亿年时，黑子与亮斑的光变信号相互抵消所致。通过分析同时期的光变与色球层时间序列，作者发现，黑子与亮斑的变异性相位差从反相（以黑子为主）转变为同相（以亮斑为主），这一转变发生在罗素数为1附近，表明约25.5亿年时，活动由黑子主导转为亮斑主导，从而解释了观测到的周期间隙。

ABSTRACT

The study of stellar activity cycles is crucial to understand the underlying dynamo and how it causes activity signatures such as dark spots and bright faculae. We study the appearance of activity signatures in contemporaneous photometric and chromospheric time series. Lomb-Scargle periodograms are used to search for cycle periods present in both time series. To emphasize the signature of the activity cycle we account for rotation-induced scatter in both data sets by fitting a quasi-periodic Gaussian process model to each observing season. After subtracting the rotational variability, cycle amplitudes and the phase difference between the two time series are obtained by fitting both time series simultaneously using the same cycle period. We find cycle periods in 27 of the 30 stars in our sample. The phase difference between the two time series reveals that the variability in fast rotating active stars is usually in anti-phase, while the variability of slowly rotating inactive stars is in phase. The photometric cycle amplitudes are on average six times larger for the active stars. The phase and amplitude information demonstrates that active stars are dominated by dark spots, whereas less active stars are dominated by bright faculae. We find the transition from spot to faculae domination at the Vaughan-Preston gap, and around a Rossby number equal to one. We conclude that faculae are the dominant ingredient of stellar activity cycles at ages >2.55 Gyr. The data further suggest that the Vaughan-Preston gap can not explain the previously detected dearth of Kepler rotation periods between 15-25 days. Nevertheless, our results led us to propose an explanation for the rotation period dearth to be due to the non-detection of periodicity caused by the cancellation of dark spots and bright faculae at 800 Myr.

研究动机与目标

• 通过分析同时期的光变与色球层时间序列，探究恒星活动周期的物理起源。
• 确定在不同旋转周期与活动水平下，暗斑还是亮斑主导恒星活动周期。
• 解决Kepler数据中15–25天中等旋转周期稀少现象这一长期存在的谜题。
• 检验Vaughan-Preston间隙是否能完全解释观测到的周期间隙，或是否存在其他机制。

提出的方法

• 使用Lomb-Scargle周期图检测光变与色球层（S指数）时间序列中的周期。
• 应用准周期高斯过程模型，以考虑自转调制并分离与周期相关的变异性。
• 使用相同的周期同时拟合光变与色球层时间序列，以测量振幅与相位差。
• 计算光变与色球层变异性之间的相位差，以推断主导活动特征（黑子或亮斑）。
• 以罗素数作为湍流周转时间的代理，比较不同恒星年龄与活动水平下的相位行为。
• 分析了30颗具有长期基线光变与色球层数据的恒星，以识别相位与振幅的系统性趋势。

实验结果

研究问题

• RQ1 15–25天之间的Kepler旋转周期稀少现象的成因是什么？是否与Vaughan-Preston间隙有关？
• RQ2 光变与色球层变异性之间的相位关系如何随恒星自转周期与活动水平变化？
• RQ3 在恒星生命周期的哪个阶段，亮斑成为光变变异性主导因素？
• RQ4 光变与色球层周期之间的相位差能否区分以黑子为主或以亮斑为主的活动？
• RQ5 罗素数在决定恒星活动周期中从黑子主导转向亮斑主导的过程中起什么作用？

主要发现

• 在30颗恒星中检测到了27个周期，证实了在广泛恒星类型中长期活动周期的存在。
• 快速自转、活动性强的恒星表现出光变与色球层信号的反相变异性，表明暗斑主导其活动。
• 慢速自转、活动性弱的恒星表现出同相变异性，表明亮斑主导其活动周期。
• 从黑子主导转向亮斑主导的转变发生在罗素数约为1时，对应恒星年龄约25.5亿年。
• 活跃恒星的光变周期振幅平均为六倍，证实了以黑子为主系统的光变调制更强。
• 15–25天中等旋转周期的稀少现象更可能由约8亿年时黑子与亮斑信号的抵消解释，而非Vaughan-Preston间隙。