[论文解读] Magnetic activity in the HARPS M-dwarf sample. The rotation-activity relationship for very low-mass stars through R'HK

N. Astudillo-Defru, X. Delfosse+4 more|HAL (Le Centre pour la Communication Scientifique Directe)|Oct 27, 2016

Stellar, planetary, and galactic studies参考文献 74被引用 4

一句话总结

本研究利用高分辨率HARPS光谱学对低质量及超低质量恒星（M型矮星）的$R^{\prime}_{HK}$指数进行了校准，建立了可靠的恒星自转-活动关系。研究发现，当自转周期短于10天时，$R^{\prime}_{HK}$在约$10^{-4}$处达到饱和，且在0.6 M$_\odot$以下质量范围内活动水平随质量降低而下降，同时显示出与平均活动水平相关的强烈短期变异性。

ABSTRACT

Atmospheric magnetic fields in stars with convective envelopes heat stellar chromospheres. Starting with the historical Mount Wilson monitoring program, CaH&K lines have been widely used to trace stellar magnetic activity, and as a proxy for rotation period and consequently for stellar age. Monitoring stellar activity has also become essential in filtering out false-positives due to magnetic activity in extra-solar planet surveys. The Ca H&Kemission is traditionally quantified through the R'HK-index, which compares the chromospheric flux in the doublet to the overall bolometric flux of the star. Much work has been done to characterize this index for FGK-dwarfs, but M-dwarfs were left out of these analyses and no calibration of their Ca ii H&K emission to an R'HK exists to date. We set out to characterize the magnetic activity of the low and very low-mass stars by providing a calibration of the R'HK-index that extends to the realm of M-dwarfs, and by evaluating the relation between R'HK and the rotation period. We calibrated the bolometric and photospheric factors for M-dwarfs to properly transform the S-index into the R'HK. We monitored magnetic activity through the Ca ii H&K emission lines in the HARPS M-dwarf sample. The R'HK index, like the fractional X-ray luminosity L_X/Lbol, shows a saturated correlation with rotation, with saturation setting in around a ten days rotation period. Above that period, slower rotators show weaker Ca ii activity, as expected. Under that period, the R'HK index saturates to ~10^-4. Stellar mass modulates the Ca ii activity, with R'HK showing a constant basal activity above 0.6Msun and then decreasing with mass between 0.6Msun and the fully-convective limit of 0.35Msun. Short-term variability of the activity correlates with its mean level, stars with higher R'HK index show larger R'HK variability, as previously observed for earlier spectral types.

研究动机与目标

• 对低质量及超低质量恒星（M型矮星）的$R^{\prime}_{HK}}$指数进行校准，此前该领域尚无相关校准。
• 建立M型矮星中$R^{\prime}_{HK}}$与自转周期之间的关系。
• 量化恒星质量如何调制M型矮星的色球层活动。
• 评估$R^{\prime}_{HK}}$的短期变异性及其与平均活动水平的相关性。
• 通过提供可靠的活动代理指标，支持系外行星探测，以过滤M型恒星系统中的径向速度抖动。

提出的方法

• 校准了总辐射和光球层因子，将S指数（Ca ii H和K线的通量相对于附近连续谱的比值）转换为M型矮星的$R^{\prime}_{HK}}$。
• 利用HARPS高分辨率光谱仪对一组M型矮星进行观测，测量Ca ii H和K发射线。
• 从每颗恒星的多次观测中计算$R^{\prime}_{HK}}$的中值及其离散度。
• 通过周期图分析并结合数据中的自转-活动关系（公式12）推导出自转周期。
• 应用对恒星参数（如V-K色指数、视差）的修正，以确保$R^{\prime}_{HK}}$转换的准确性。
• 通过比较样本中$R^{\prime}_{HK}}$的离散度与平均值，分析其变异性。

实验结果

研究问题

• RQ1 在自转周期短于10天的M型矮星中，$R^{\prime}_{HK}}$指数如何表现？
• RQ2 在0.35–0.6 M$_\odot$质量范围内，$R^{\prime}_{HK}}$对恒星质量的依赖关系如何？
• RQ3 $R^{\prime}_{HK}}$是否在高活动水平下出现饱和？如果是，其饱和值是多少？
• RQ4 $R^{\prime}_{HK}}$的短期变异性如何与其中均活动水平相关？
• RQ5 $R^{\prime}_{HK}}$指数能否在M型恒星的径向速度巡天中可靠地用作磁活动的代理指标？

主要发现

• 当自转周期短于10天时，$R^{\prime}_{HK}}$在约$10^{-4}$处达到饱和，表明色球层活动存在最大水平。
• 对于质量大于0.6 M$_\odot$的恒星，$R^{\prime}_{HK}}$保持在恒定的基底活动水平，而在0.6 M$_\odot$至0.35 M$_\odot$完全对流极限之间，活动水平随质量减小而降低。
• 平均$R^{\prime}_{HK}}$值较高的恒星表现出更大的$R^{\prime}_{HK}}$短期变异性，证实了活动水平与变异性幅度之间存在相关性。
• $R^{\prime}_{HK}}$与自转周期之间的相关性呈现饱和状态，与更大质量恒星的行为一致。
• 对M型矮星$R^{\prime}_{HK}}$的校准，使得在系外行星探测任务中能够更有效地过滤由活动引起的径向速度抖动。
• 电子附录中的表6提供了HARPS M型矮星样本的完整$R^{\prime}_{HK}}$测量值、S指数值及推导出自转周期。