A solar-like magnetic cycle on the mature K-dwarf 61 Cygni A (HD 201091)
A solar-like magnetic cycle on the mature K-dwarf 61 Cygni A (HD 201091)
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DOI：
10.1051/0004-6361/201628262
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发表时间：
2016-06
影响因子：
6.5
通讯作者：
S. Saikia;S. Jeffers;J. Morin;P. Petit;P. Petit;C. Folsom;S. Marsden;J. Donati;J. Donati;R. H. Cameron;J. C. Hall;V. Perdelwitz;A. Reiners;A. Vidotto;A. Vidotto
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文献类型：
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作者：
S. Saikia;S. Jeffers;J. Morin;P. Petit;P. Petit;C. Folsom;S. Marsden;J. Donati;J. Donati;R. H. Cameron;J. C. Hall;V. Perdelwitz;A. Reiners;A. Vidotto;A. Vidotto
对冷恒星磁循环的长期监测是了解结构与太阳相似的恒星中磁场的发电机产生和放大如何发生的关键诊断。我们使用分光偏振观测研究了 61 Cyg A 在其活动周期内可能的磁周期的时间演化，该磁周期由其大尺度磁场确定，并将其与太阳情况进行比较。我们使用塞曼多普勒成像（ZDI）来重建跨越九年时间跨度的多个观测时期的大规模磁几何。我们研究了大尺度场的不同组成部分的时间演化，并通过测量三种不同色球指标的通量将其与其色球活动的演化进行比较：Ca II H&K、H-alpha 和 Ca II 红外三重线。我们还使用 XMM-Newton 观测将我们的结果与恒星日冕活动进行比较。 61 Cyg A 的大尺度磁几何结构在极向场和环形场分量中都表现出极性反转，与色球活动周期同相。我们还探测到微弱的类太阳差分旋转，其剪切水平与太阳相似。在我们九年的观测时间跨度中，61 Cyg A 在其大尺度场几何结构中表现出类似太阳的变化，因为它从最小活动演化到最大活动，反之亦然。在 2007.59 纪元的活动最小值期间，ZDI 重建了一个简单的偶极几何结构，该几何结构在接近 2010.55 纪元的活动最大值时变得更加复杂。径向场在纪元 2013.61 翻转极性并恢复到简单的几何形状。该场是强偶极场，场的偶极子分量的演变让人想起太阳的行为。大尺度场的极性反转表明磁周期与色球周期和日冕周期同相。
The long-term monitoring of magnetic cycles in cool stars is a key diagnostic in understanding how dynamo generation and amplification of magnetic fields occur in stars similar in structure to the Sun. We investigate the temporal evolution of a possible magnetic cycle, determined from its large-scale field, of 61 Cyg A over its activity cycle using spectropolarimetric observations and compare it to the solar case. We use Zeeman Doppler imaging (ZDI) to reconstruct the large-scale magnetic geometry over multiple observational epochs spread over a time span of nine years. We investigate the time evolution of the different components of the large-scale field and compare it with the evolution of its chromospheric activity by measuring the flux in three different chromospheric indicators: Ca II H&K, H-alpha and Ca II infra red triplet lines. We also compare our results with the star's coronal activity using XMM-Newton observations. The large-scale magnetic geometry of 61 Cyg A exhibits polarity reversals in both poloidal and toroidal field components, in phase with the chromospheric activity cycle. We also detect weak solar-like differential rotation with a shear level similar to the Sun. During our observational time span of nine years, 61 Cyg A exhibits solar-like variations in its large-scale field geometry as it evolves from minimum activity to maximum activity and vice versa. During its activity minimum in epoch 2007.59, ZDI reconstructs a simple dipolar geometry which becomes more complex close to activity maximum in epoch 2010.55. The radial field flips polarity and reverts back to a simple geometry in epoch 2013.61. The field is strongly dipolar and the evolution of the dipole component of the field is reminiscent of the solar behaviour. The polarity reversal of the large-scale field indicates a magnetic cycle that is in phase with the chromospheric and coronal cycle.