arXiv 1306.21762013-06-10astro-ph.SR

一种快速可靠的光度测量恒星自转差的方法

A fast and reliable method to measure stellar differential rotation from photometric data

Timo Reinhold, Ansgar Reiners

备注：: accepted by A&A
DOI：: 10.1051/0004-6361/201321161

摘要：

在恒星表面不同纬度的同向旋转斑点产生周期性的光度变化，可以作为探测差异旋转（DR）的有用代理。DR是太阳发电机的主要组成部分，但恒星DR的观测相当稀少。针对开普勒空间望远镜，我们有兴趣利用恒星的光度信息检测DR，并发展一种快速的方法来确定恒星DR从光度数据。我们运行了一个大的蒙特卡罗模拟差分旋转斑点星具有非常不同的属性，以调查DR的可探测性。对于不同的噪声水平，由此产生的光变曲线预白化使用Lomb-Scargle周期图，以获得参数的全球正弦拟合检测周期性。我们展示了在什么条件下DR可以成功地从光度数据中检测到，在这种情况下，光变曲线提供的恒星自转规律的信息不足，甚至是误导。在我们的模拟中，最重要的周期P1_{out}可以检测到的96.2%的所有光变曲线。在我们的模型中，检测到接近P1_{out}的第二周期是DR的签名。在无噪声的情况下，有64.2%的恒星存在这样的周期，计算两个不同黑子的纬向切变α_{out}，并与已知的原始黑子自转速率进行比较，结果表明，实际值平均低3.2%。比较总赤道-极切变$α$和$α_{out}$，我们发现$α$被低估了8.8%，特别是对于$α$ < 6%的恒星，DR的检测是具有挑战性的。最后，我们将我们的方法应用于四个差分旋转开普勒星，并找到与详细建模的结果密切一致。我们的方法是能够测量恒星自转周期和检测DR相对较高的精度，适用于大数据集。

英文摘要：

Co-rotating spots at different latitudes on the stellar surface generate periodic photometric variability and can be useful proxies to detect Differential Rotation (DR). DR is a major ingredient of the solar dynamo but observations of stellar DR are rather sparse. In view of the Kepler space telescope we are interested in the detection of DR using photometric information of the star, and to develop a fast method to determine stellar DR from photometric data. We ran a large Monte-Carlo simulation of differentially rotating spotted stars with very different properties to investigate the detectability of DR. For different noise levels the resulting light curves are prewhitened using Lomb-Scargle periodograms to derive parameters for a global sine fit to detect periodicities. We show under what conditions DR can successfully be detected from photometric data, and in which cases the light curve provides insufficient or even misleading information on the stellar rotation law. In our simulations, the most significant period P1_{out} could be detected in 96.2% of all light curves. Detection of a second period close to P1_{out} is the signature of DR in our model. For the noise-free case, in 64.2% of all stars such a period was found. Calculating the measured latitudinal shear of two distinct spots α_{out}, and comparing it to the known original spot rotation rates shows that the real value is on average 3.2% lower. Comparing the total equator-to-pole shear $α$ to $α_{out}$ we find that $α$ is underestimated by 8.8%, esp. the detection of DR for stars with $α$ < 6% is challenging. Finally, we apply our method to four differentially rotating Kepler stars and find close agreement with results from detailed modeling. Our method is capable of measuring stellar rotation periods and detecting DR with relatively high accuracy and is suitable for large data sets.

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