[论文解读] A quantitative analysis of stellar activity based on CoRoT photometric data

J. C. Hulot, F. Baudin+2 more|arXiv (Cornell University)|Apr 12, 2011

Astronomical Observations and Instrumentation参考文献 1被引用 3

一句话总结

本研究基于CoRoT光度光 light curves，提出了一种基于傅里叶变换的活动指数，用于量化430颗主序星的恒星微变异性。通过在低频功率谱上使用广义半洛伦兹拟合，将该指数与罗斯比数相关联，发现较冷恒星的活动水平更高，且特征 timescale 随自转周期增加而增加。

ABSTRACT

The CoRoT satellite has made available high precision photometric observations of a large number of stars of different spectral types. Continuous photometric time series allow the characterization of stellar microvariability in a systematic way. We determine an index indicating the level of activity, derived from photometric data, for a large sample of stars with different color temperatures. We also assess to what extent this index can be related to an estimated Rossby number for stars whose rotation period can be estimated. We also estimate a characteristic lifetime of the surface heterogeneities. Our work is based on the Fourier analysis of stellar light curves. We analyzed the Fourier power spectra of 430 selected light curves obtained by CoRoT during three observation runs. The low-frequency contribution of the stellar variability is modelled by a "generalized semi-lorentzian" profile. An activity index is derived from the fitted amplitude and width of the semi-lorentzian model. Some of the Fourier spectra exhibit a rotational modulation which enables the determination of the rotation period. In addition, a convective turnover time is derived from a grid of stellar models, so that a Rossby number can be estimated. A characteristic lifetime of the phenomena causing the observed power at low frequency is assessed from the fitted model of the power spectrum and is compared to the rotation period. Higher values of the microvariability index are observed among the coolest stars from our sample. 28 light curves show a clear rotational modulation. The estimated Rossby number of most of the observed stars with a rotational modulation is less than 1. The activity index decreases with increasing Rossby number. The quality of the CoRoT data enables the determination of the characteristic lifetime of active structures. It is shown to increase with the rotation period.

研究动机与目标

• 从高精度CoRoT光度光 light curves 中开发一种基于傅里叶变换的定量活动指数，适用于大样本主序星。
• 利用光曲线调制估算的自转周期，评估所推导活动指数与罗斯比数之间的关系。
• 估计导致低频光度变异性表面异质性的特征寿命。
• 研究恒星颜色温度、自转及湍流周转时间对活动水平的影响。
• 评估在存在观测偏差和活动周期的情况下，单参数活动代理（如罗斯比数）的局限性。

提出的方法

• 对430条CoRoT光曲线进行傅里叶分析，使用广义半洛伦兹轮廓对低频功率过剩进行建模。
• 通过拟合半洛伦兹函数的振幅与宽度，从功率谱中推导出定量活动指数。
• 通过低频峰识别光曲线中的自转调制，以估算自转周期。
• 利用恒星模型网格计算湍流周转时间，并推导罗斯比数（Ro = P_rot / τ_conv）。
• 根据拟合的洛伦兹模型参数，估计活动结构的特征寿命。
• 在不同光谱型中比较所推导活动指数与颜色温度、罗斯比数及自转周期的关系。

实验结果

研究问题

• RQ1 傅里叶基活动指数在主序星上与恒星颜色温度的相关性如何？
• RQ2 对于具有可测量自转周期的恒星，所推导活动指数在多大程度上可与罗斯比数关联？
• RQ3 表面异质性特征的特征寿命与恒星自转周期之间存在何种关系？
• RQ4 观测偏差（如短 timescale、视线几何）如何影响自转调制与活动指数的检测？
• RQ5 活动指数能否可靠地量化无明显自转调制的恒星的微变异性？

主要发现

• 在样本中，最冷恒星的活动指数最高，与颜色温度呈明显负相关。
• 28条光曲线表现出明显的自转调制，其自转周期随颜色温度升高而减小。
• 大多数具有自转调制的恒星罗斯比数小于1，表明磁活动水平较高，与预期一致。
• 活动指数随罗斯比数增加而减小，证实了活动与发电机效率之间的反比关系。
• 活动结构的特征寿命随自转周期增加而增长，尽管由于表面特征差异（如黑子与亮斑）存在散射。
• 该方法即使在无调制恒星中也能成功量化活动，但未解析的低频峰可能使慢速旋转的M型矮星的活动被高估。