Long-term magnetic activity of a sample of M-dwarf stars from the HARPS program I. Comparison of activity indices
Long-term magnetic activity of a sample of M-dwarf stars from the HARPS program I. Comparison of activity indices
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HARPS 计划 M 矮星样本的长期磁活动 I. 活动指数比较
DOI：
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发表时间：
2011
期刊：
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通讯作者：
S. Udry
中科院分区：
文献类型：
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作者：
J. G. D. Silva;N. Santos;X. Bonfils;X. Delfosse;T. Forveille;S. Udry
上下文。寻找类似地球的太阳系外行星正在成为现实，但随着测量到的径向速度水平达到亚毫秒−1，能够隐藏这些行星信号的恒星固有噪声源变得更加重要。目标。已知其他恒星也有类似于太阳的磁周期。这些活度变化与观测到的径向速度之间的关系仍然没有得到令人满意的理解。我们之前研究了K矮星的活动周期和长期速度变化之间的关系，现在我们将其扩展到主序列的低端。在第一篇论文中，我们的目标是评估主序列低端的长期活动变化，并考虑到行星搜索的角度。方法。我们使用了来自HARPS m -矮行星搜索计划的30颗M0 - M5.5恒星样本，观测的中位数时间跨度为5.2年。我们根据Ca ii Ha和K， Hα，H i D3和Na i D1和D2系计算了色球活性指标。所有的数据都进行了分组，以平均不期望的影响，如旋转调制的大气不均匀性。我们搜索了每个指标的长期变异性，并确定了它们之间的相关性。结果。在我们的恒星样本中，只有一小部分（分别为39%、33%和37%）的S cai、H α、a和Nai指数显示出显著的变化，而只有10%的恒星在He i指数中显示出显著的变化。因此，我们得出结论，这个指数是一个很差的活动指标，至少对这类恒星来说是这样。虽然在最活跃的恒星中，Hα与S CaII具有良好的相关性，但当活动水平降低时，相关性就消失了。这一结果似乎表明Ca ii−Hα的相关性取决于恒星的活动水平。Na i线与我们使用的低活动水平恒星的S CaII指数非常相关，因此是早期m矮星的一个很好的色球活动代理。因此，我们强烈建议使用Nai活性指数，因为钠谱线光谱区域的信噪比总是高于钙谱线。
Context. The search for extra-solar planets similar to Earth is becoming a reality, but as the level of the measured radial-velocity reaches the sub-ms −1 , stellar intrinsic sources of noise capable of hiding the signal of these planets from scrutiny become more important. Aims. Other stars are known to have magnetic cycles similar to that of the Sun. The relationship between these activity variations and the observed radial-velocity is still not satisfactorily understood. Following our previous work, which studied the correlation between activity cycles and long-term velocity variations for K dwarfs, we now expand it to the lower end of the main sequence. In this first paper our aim is to assess the long-term activity variations in the low end of the main sequence, having in mind a planetary search perspective. Methods. We used a sample of 30 M0‐M5.5 stars from the HARPS M-dwarf planet search program with a median timespan of observations of 5.2 years. We computed chromospheric activity indicators based on the Ca ii Ha nd K, Hα ,H ei D3, and Na i D1 and D2 lines. All data were binned to average out undesired effects such as rotationally modulated atmospheric inhomogeneities. We searched for long-term variability of each index and determined the correlations between them. Results. While the S CaII ,H α ,a nd Nai indices showed significant variability for a fraction of our stellar sample (39%, 33%, and 37%, respectively), only 10% of our stars presented significant variability in the He i index. We therefore conclude that this index is a poor activity indicator at least for this type of stars. Although the Hα shows good correlation with S CaII for the most active stars, the correlation is lost when the activity level decreases. This result appears to indicate that the Ca ii−Hα correlation is dependent on the activity level of the star. The Na i lines correlate very well with the S CaII index for the stars with low activity levels we used, and are thus a good chromospheric activity proxy for early-M dwarfs. We therefore strongly recommend the use of the Nai activity index because the signal-to-noise ratio in the sodium lines spectral region is always higher than for the calcium lines.