磁激励线驱动恒星风的动力学模拟：III。Angular Momentum损耗和旋转降速
Dynamical Simulations of Magnetically Channeled Line-Driven Stellar Winds: III. Angular Momentum Loss and Rotational Spindown
摘要：
我们研究的角动量损失和相关的旋转自旋与线驱动的恒星风和旋转排列的偶极磁场的磁性热星。我们的分析是基于我们以前的2-D数值MHD模拟研究，研究风，场和旋转之间的相互作用作为两个无量纲参数的函数，一个表征风磁约束（$η_{\ast} \equiv B_{eq}^{2} R_{\ast}^{2}/{\dot M} v_{\infty}$），和其他的比率（$W \equiv V_{rot}/V_{orb}$）的恒星旋转临界（轨道）速度。我们比较和对比的2-D，随时间变化的角动量损失的偶极子模型的热星风与经典的1-D稳态分析的韦伯和戴维斯（WD），谁使用了理想化的电磁场模型的角动量损失的太阳风。尽管存在差异，我们发现，总的角动量损失${\dot J}$在两个立体角和时间的平均密切遵循一般WD标度${\dot J} = (2/3) {\dot M} ΩR_{A}^{2}$，其中${\dot M}$是质量损失率，$Ω$是恒星的角速度，和$R_{A}$是一个特征阿尔文半径。然而，这里的一个关键区别是，对于偶极场，这个阿尔文半径具有强场标度$R_{A}/R_{\ast} \approx η_{\ast}^{1/4}$，而不是弱场的标度$R_{A}/R_{\ast} \sim \sqrt{η_{\ast}}$。这导致了一个更慢的恒星自旋时间，在偶极情况下的尺度为$τ_{spin} = τ_{mass} 1.5k/\sqrt{η_{\ast}}$，其中$τ_{mass} \equiv M/{\dot M}$是特征质量损失时间，$k$是恒星惯性矩的无量纲因子。完整的数值标度关系，我们引用了几个已知的磁性大质量恒星的典型的自旋时间为100万年。
英文摘要：
We examine the angular momentum loss and associated rotational spindown for magnetic hot stars with a line-driven stellar wind and a rotation-aligned dipole magnetic field. Our analysis here is based on our previous 2-D numerical MHD simulation study that examines the interplay among wind, field, and rotation as a function of two dimensionless parameters, one characterizing the wind magnetic confinement ($η_{\ast} \equiv B_{eq}^{2} R_{\ast}^{2}/{\dot M} v_{\infty}$), and the other the ratio ($W \equiv V_{rot}/V_{orb}$) of stellar rotation to critical (orbital) speed. We compare and contrast the 2-D, time variable angular momentum loss of this dipole model of a hot-star wind with the classical 1-D steady-state analysis by Weber and Davis (WD), who used an idealized monopole field to model the angular momentum loss in the solar wind. Despite the differences, we find that the total angular momentum loss ${\dot J}$ averaged over both solid angle and time follows closely the general WD scaling ${\dot J} = (2/3) {\dot M} ΩR_{A}^{2}$, where ${\dot M}$ is the mass loss rate, $Ω$ is the stellar angular velocity, and $R_{A}$ is a characteristic Alfvén radius. However, a key distinction here is that for a dipole field, this Alfvén radius has a strong-field scaling $R_{A}/R_{\ast} \approx η_{\ast}^{1/4}$, instead of the scaling $R_{A}/R_{\ast} \sim \sqrt{η_{\ast}}$ for a monopole field. This leads to a slower stellar spindown time that in the dipole case scales as $τ_{spin} = τ_{mass} 1.5k/\sqrt{η_{\ast}}$, where $τ_{mass} \equiv M/{\dot M}$ is the characteristic mass loss time, and $k$ is the dimensionless factor for stellar moment of inertia. The full numerical scaling relation we cite gives typical spindown times of order 1 Myr for several known magnetic massive stars.
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