类太阳恒星中的环向和极向磁场：旋转阈值
Toroidal versus poloidal magnetic fields in Sun-like stars: a rotation threshold
摘要：
从一组恒星的偏振光谱观测，我们报告的表面磁场的检测在一个样本的四个太阳型恒星，即HD 73350，HD 76151，HD 146233（18天蝎座）和HD 190771。假设观测到的偏振信号的变化是由恒星自转控制的，我们建立了我们的目标的旋转周期，其值范围从8.8 d（HD 190771）到22.7 d（HD 146233）。除了自转之外，所选天体的基本参数与太阳非常接近，这使得该样本成为研究自转对类太阳恒星磁特性的具体影响的实用基础。我们重建的大规模磁几何的目标作为一个低阶（l<10）球谐展开的表面磁场。从这组磁图中，我们得出两个主要结论。(a)大尺度磁场的磁能随旋转速率的增加而增加。色球辐射随平均磁场的增加比在太阳中观测到的要平缓。由于色球通量也对小于极化信号的磁性元素敏感，这一观察表明，随着旋转的增加，更大比例的表面磁能被储存在大尺度上。(b)而磁场主要是极向的低转速，更快速的转子主机在其表面场的大规模的环形分量。根据我们的观测，我们推断，一个低于12天的旋转周期是必要的环向磁能占主导地位的极向分量。
英文摘要：
From a set of stellar spectropolarimetric observations, we report the detection of surface magnetic fields in a sample of four solar-type stars, namely HD 73350, HD 76151, HD 146233 (18 Sco) and HD 190771. Assuming that the observed variability of polarimetric signal is controlled by stellar rotation, we establish the rotation periods of our targets, with values ranging from 8.8 d (for HD 190771) to 22.7 d (for HD 146233). Apart from rotation, fundamental parameters of the selected objects are very close to the Sun's, making this sample a practical basis to investigate the specific impact of rotation on magnetic properties of Sun-like stars. We reconstruct the large-scale magnetic geometry of the targets as a low-order (l<10) spherical harmonics expansion of the surface magnetic field. From the set of magnetic maps, we draw two main conclusions. (a) The magnetic energy of the large-scale field increases with rotation rate. The increase of chromospheric emission with the mean magnetic field is flatter than observed in the Sun. Since the chromospheric flux is also sensitive to magnetic elements smaller than those contributing to the polarimetric signal, this observation suggests that a larger fraction of the surface magnetic energy is stored in large scales as rotation increases. (b) Whereas the magnetic field is mostly poloidal for low rotation rates, more rapid rotators host a large-scale toroidal component in their surface field. From our observations, we infer that a rotation period lower than ~12 days is necessary for the toroidal magnetic energy to dominate over the poloidal component.
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