Exploring the magnetic field complexity in M dwarfs at the boundary to full convection
Exploring the magnetic field complexity in M dwarfs at the boundary to full convection
复制标题
探索M矮星在完全对流边界处的磁场复杂性
DOI：
10.1051/0004-6361/201322136
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发表时间：
2014
影响因子：
6.5
通讯作者：
Piskunov
中科院分区：
文献类型：
--
作者：
Shulyak;Reiners;Seemann;Kochukhov;Piskunov
磁场在低质量恒星的形成和演化中起着关键作用，但产生这些磁场的发电机机制却知之甚少。由于冷恒星光谱的复杂性和磁场的微妙特征，测量冷恒星的磁性是一项复杂的任务。目的在详细的光谱合成的基础上，定量测量了gj388、gj729、gj285和gj406四颗已知的M矮星表面磁场的强度和复杂性，它们分布在部分对流和完全对流恒星边界附近的质量区。在ESO的甚大望远镜上，利用CRIRES获得了高分辨率（R= 100,000）、高信噪比（高达400）的近红外斯托克光谱，覆盖了FeH翼-福特跃迁1μm和Na i线2.2μm的区域。方法利用改良版的分子塞曼文库（MZL）计算FeH系的land<s:1> g因子。我们用Synmast代码进行磁谱合成来确定磁场的分布。我们测试了两种不同的磁几何形状来探测磁场取向效应的影响。结果我们的分析证实，FeH谱线是M型低质量恒星表面磁场的优秀指标，特别是与Na i谱线的谱线相比，Na i谱线受到水线的严重影响，并且在连续统归一化方面存在问题。这四颗恒星的场分布都有三组不同的场分量，数据既不符合不同场强的平滑分布，也不符合覆盖整颗恒星的平均场强。我们发现与其他三个目标相比，gj285的场分布有细微的差异。gj285的平均田间分量最高，为3.5 kG，最大田间分量最强，为7 ~ 7.5 kG。在我们的样本中，最大局部场强似乎与旋转速率相关。虽然平均场强是饱和的，但我们样本中的最大局部场强没有显示饱和的证据。结论部分对流星和完全对流星的场分布无明显差异。有证据表明磁场分布与其他三颗不同的那颗恒星是最活跃的恒星（即具有x射线光度和平均表面磁场），旋转相对较快。一种可能的解释是，旋转决定了表面磁场的分布，即使在平均磁场已经饱和的恒星中，局部磁场强度也会随着旋转而增加。
ContextMagnetic fields play a pivotal role in the formation and evolution of low-mass stars, but the dynamo mechanisms generating these fields are poorly understood. Measuring cool star magnetism is a complicated task because of the complexity of cool star spectra and the subtle signatures of magnetic fields.AimsBased on detailed spectral synthesis, we carry out quantitative measurements of the strength and complexity of surface magnetic fields in the four well-known M dwarfs GJ 388, GJ 729, GJ 285, and GJ 406 that populate the mass regime around the boundary between partially and fully convective stars. Very high-resolution (R= 100 000), high signal-to-noise (up to 400), near-infrared StokesIspectra were obtained with CRIRES at ESO’s Very Large Telescope covering regions of the FeH Wing-Ford transitions at 1μm and Na i lines at 2.2μm.MethodsA modified version of the Molecular Zeeman Library (MZL) was used to compute Landé g-factors for FeH lines. We determined the distribution of magnetic fields by magnetic spectral synthesis performed with the Synmast code. We tested two different magnetic geometries to probe the influence of field orientation effects.ResultsOur analysis confirms that FeH lines are excellent indicators of surface magnetic fields in low-mass stars of type M, particularly in comparison to profiles of Na i lines that are heavily affected by water lines and that suffer problems with continuum normalization. The field distributions in all four stars are characterized by three distinct groups of field components, and the data are consistent neither with a smooth distribution of different field strengths nor with one average field strength covering the full star. We find evidence of a subtle difference in the field distribution of GJ 285 compared to the other three targets. GJ 285 also has the highest average field of 3.5 kG and the strongest maximum field component of 7–7.5 kG. The maximum local field strengths in our sample seem to be correlated with rotation rate. While the average field strength is saturated, the maximum local field strengths in our sample show no evidence of saturation.ConclusionsWe find no difference between the field distributions of partially and fully convective stars. The one star with evidence of field distribution different from the other three is the most active star (i.e. with X-ray luminosity and mean surface magnetic field) rotating relatively fast. A possible explanation is that rotation determines the distribution of surface magnetic fields, and that local field strengths grow with rotation even in stars in which the average field is already saturated.