arXiv 1509.087512015-09-29astro-ph.SR

磁场几何形状的变化会导致风与活动关系的破裂吗？

Could a change in magnetic field geometry cause the break in the wind-activity relation?

A. A. Vidotto, J. -F. Donati, M. Jardine, V. See, P. Petit, I. Boisse, S. Boro Saikia, E. Hebrard, S. V. Jeffers, S. C. Marsden, J. Morin

备注：: 7 pages, 3 figures, 4 tables, accepted to MNRAS Letters
DOI：: 10.1093/mnrasl/slv147

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摘要：

Wood等人提出，对于具有$F_x \lesssim F_{x,6} \equiv 10^6$ erg cm $^{-2}$ s $^{-1}$的矮星，质量损失率是X射线通量（$\dot{M} \propto F_x^{1.34}$）的函数。然而，更活跃的恒星并不遵循这种关系。这些作者认为，$F_{x,6}$的断裂可能是由磁场拓扑结构的显著变化引起的，这种变化会抑制恒星风的产生。在这里，我们调查这一假设，通过分析在伍德等人的样本，其表面磁场重建通过塞曼多普勒成像（ZDI）的恒星。虽然在$\dot{M}$-$F_x$关系中的类太阳异常值具有更高的分数环向磁能，但我们没有发现$F_{x,6}$处磁拓扑结构急剧转变的证据。为了证实这一点，需要在断裂两侧进行进一步的风力测量和ZDI观测。由于活跃的恒星可以在具有高环向场和高极向场的状态之间跳跃，我们预计具有$F_x \gtrsim F_{x,6}$的恒星在磁场拓扑中存在显著的散射。这加强了多历元ZDI观测的重要性。最后，我们表明，有一个$F_x$和磁能之间的相关性，这意味着$\dot{M}$-磁能关系具有相同的定性行为作为原来的$\dot{M}$-$F_x$关系。在任何$F_x$中均未观察到断裂-磁能

英文摘要：

Wood et al suggested that mass-loss rate is a function of X-ray flux ($\dot{M} \propto F_x^{1.34}$) for dwarf stars with $F_x \lesssim F_{x,6} \equiv 10^6$ erg cm$^{-2}$ s$^{-1}$. However, more active stars do not obey this relation. These authors suggested that the break at $F_{x,6}$ could be caused by significant changes in magnetic field topology that would inhibit stellar wind generation. Here, we investigate this hypothesis by analysing the stars in Wood et al's sample that had their surface magnetic fields reconstructed through Zeeman-Doppler Imaging (ZDI). Although the solar-like outliers in the $\dot{M}$ -- $F_x$ relation have higher fractional toroidal magnetic energy, we do not find evidence of a sharp transition in magnetic topology at $F_{x,6}$. To confirm this, further wind measurements and ZDI observations at both sides of the break are required. As active stars can jump between states with highly toroidal to highly poloidal fields, we expect significant scatter in magnetic field topology to exist for stars with $F_x \gtrsim F_{x,6}$. This strengthens the importance of multi-epoch ZDI observations. Finally, we show that there is a correlation between $F_x$ and magnetic energy, which implies that $\dot{M}$ -- magnetic energy relation has the same qualitative behaviour as the original $\dot{M}$ -- $F_x$ relation. No break is seen in any of the $F_x$ -- magnetic energy

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