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arXiv 1402.1874astro-ph.SRastro-ph.HEastro-ph.IM
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不同自转速度类太阳恒星磁活动的发电机模型

A dynamo model of magnetic activity in solar-like stars with different rotational velocities

Bidya Binay Karak, Leonid L. Kitchatinov, Arnab Rai Choudhuri

备注：
Accepted for publication in ApJ
发表信息：
ApJ, 791, 59 (2014)
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摘要：

我们试图对类太阳恒星的Ca II H/K发射和X射线发射随Rossby数的减小而增加的观测结果提供定量的理论解释（即，更快的旋转）。假设这些排放是由类似于太阳黑子周期的磁循环引起的，我们构建了不同旋转周期的$1M_{\odot}$恒星的通量输运发电机模型。我们首先计算的微分旋转和纬向环流这些恒星内部的平均场流体动力学模型。然后这些被替换到我们的发电机代码中以产生周期解。我们发现，无量纲振幅$f_m$的环形通量通过恒星的旋转周期的减少而增加。如果我们假设辐射是$f_m$的3-4次方，观测数据就可以匹配。假设Babcock-Leighton机制随着旋转的增加而饱和，我们可以为在低Rossby数下观察到的发射饱和提供一个解释。我们的模型的主要失败之处在于，它预测的磁循环周期随着旋转速率的增加而增加，这与观测发现的相反。我们的许多计算都是基于磁浮力使磁通量管从对流区底部径向上升的假设。考虑到科里奥利力使磁通量管转向平行于快速旋转的恒星的旋转轴上升，结果并没有发生质的变化。

英文摘要：

We attempt to provide a quantitative theoretical explanation for the observations that Ca II H/K emission and X-ray emission from solar-like stars increase with decreasing Rossby number (i.e., with faster rotation). Assuming that these emissions are caused by magnetic cycles similar to the sunspot cycle, we construct flux transport dynamo models of $1M_{\odot}$ stars rotating with different rotation periods. We first compute the differential rotation and the meridional circulation inside these stars from a mean-field hydrodynamics model. Then these are substituted in our dynamo code to produce periodic solutions. We find that the dimensionless amplitude $f_m$ of the toroidal flux through the star increases with decreasing rotation period. The observational data can be matched if we assume the emissions to go as the power 3-4 of $f_m$. Assuming that the Babcock-Leighton mechanism saturates with increasing rotation, we can provide an explanation for the observed saturation of emission at low Rossby numbers. The main failure of our model is that it predicts an increase of magnetic cycle period with increasing rotation rate, which is the opposite of what is found observationally. Much of our calculations are based on the assumption that the magnetic buoyancy makes the magnetic flux tubes to rise radially from the bottom of the convection zone. On taking account of the fact that the Coriolis force diverts the magnetic flux tubes to rise parallel to the rotation axis in rapidly rotating stars, the results do not change qualitatively.

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