低质量恒星隐藏磁场的模拟
Modelling the Hidden Magnetic Field of Low-Mass Stars
摘要：
塞曼-多普勒成像是一种光谱偏振技术，用于绘制大尺度恒星表面磁场。这些地图反过来又被用来研究恒星的日冕和风的结构。然而，这种方法忽略了极化特征被抵消的任何小尺度磁通量。塞曼展宽的测量表明，表面磁通量的大部分可以用这种方法忽略。在本文中，我们评估的影响，这种“失踪的流量”的预测日冕结构和可能的速度自旋下降，由于恒星风。为此，我们创建了一个小尺度场的模型，并将其添加到12 M矮星样本的磁场的塞曼-多普勒图中。我们推断这一组合场，并确定一个流体静力学，等温日冕的结构。小尺度表面场的增加产生了一个覆盖大部分表面的低洼磁环地毯，包括太阳“冕洞”的恒星等效物，其中大尺度场被恒星风打开，因此将是X射线黑暗。我们表明，与旋转速率（所谓的“活动旋转关系”）的X射线发射措施的趋势是不受影响的小尺度字段添加时，相对于每个恒星的大尺度字段缩放。小尺度场的加入增加了表面通量;然而，控制风中质量和角动量损失的大尺度开放通量不受影响。我们的结论是，自旋下降时间和质量损失率从表面磁图计算不太可能被忽视的小尺度场显着影响。
英文摘要：
Zeeman-Doppler imaging is a spectropolarimetric technique that is used to map the large-scale surface magnetic fields of stars. These maps in turn are used to study the structure of the stars' coronae and winds. This method, however, misses any small-scale magnetic flux whose polarisation signatures cancel out. Measurements of Zeeman broadening show that a large percentage of the surface magnetic flux may be neglected in this way. In this paper we assess the impact of this 'missing flux' on the predicted coronal structure and the possible rates of spin down due to the stellar wind. To do this we create a model for the small-scale field and add this to the Zeeman-Doppler maps of the magnetic fields of a sample of 12 M dwarfs. We extrapolate this combined field and determine the structure of a hydrostatic, isothermal corona. The addition of small-scale surface field produces a carpet of low-lying magnetic loops that covers most of the surface, including the stellar equivalent of solar 'coronal holes' where the large-scale field is opened up by the stellar wind and hence would be X-ray dark. We show that the trend of the X-ray emission measure with rotation rate (the so-called 'activity-rotation relation') is unaffected by the addition of small-scale field, when scaled with respect to the large-scale field of each star. The addition of small-scale field increases the surface flux; however, the large-scale open flux that governs the loss of mass and angular momentum in the wind remains unaffected. We conclude that spin-down times and mass loss rates calculated from surface magnetograms are unlikely to be significantly influenced by the neglect of small-scale field.
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