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arXiv 0801.0440astro-ph
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吸积动力恒星风III：自旋平衡解

Accretion-Powered Stellar Winds III: Spin Equilibrium Solutions

Sean Matt, Ralph E. Pudritz

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Accepted for publication in ApJ
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摘要：

我们比较了恒星风扭矩计算在以前的工作（纸II）的自旋和自旋下降的扭矩预计产生的磁之间的相互作用缓慢旋转（$\sim 10$ %的解体）前主序星和吸积盘。这个分析表明，恒星风可以带走比可以转移到磁盘的角动量多几个数量级的角动量，只要质量流出率大于太阳风。因此，平衡自旋状态的特征在于吸积所沉积的角动量和恒星风所提取的角动量之间的平衡。我们推导出一个半解析公式，用于预测平衡自旋速率仅作为$\dot M_{\rm w} / \dot M_{\rm a}$和无量纲磁化参数$Ψ\equiv B_*^2 R_*^2 (\dot M_{\rm a} v_{\rm esc})^{-1}$之比的函数，其中$\dot M_{\rm w}$是恒星风质量流出速率，$\dot M_{\rm a}$是吸积速率，$B_*$是恒星表面磁场强度，$R_*$是恒星半径，$v_{\rm esc}$是表面逃逸速度。对于吸积前主序星的典型参数，这解释了$\dot M_{\rm w} / \dot M_{\rm a} \sim 0.1$分裂速度的$\sim 10$ %的自旋率。最后，假设恒星风是由吸积功率的一小部分驱动的，导致$\dot M_{\rm w} / \dot M_{\rm a} \la 0.6$的质量流比的上限。

英文摘要：

We compare the stellar wind torque calculated in a previous work (Paper II) to the spin-up and spin-down torques expected to arise from the magnetic interaction between a slowly rotating ($\sim 10$% of breakup) pre-main-sequence star and its accretion disk. This analysis demonstrates that stellar winds can carry off orders of magnitude more angular momentum than can be transferred to the disk, provided that the mass outflow rates are greater than the solar wind. Thus, the equilibrium spin state is simply characterized by a balance between the angular momentum deposited by accretion and that extracted by a stellar wind. We derive a semi-analytic formula for predicting the equilibrium spin rate as a function only of the ratio of $\dot M_{\rm w} / \dot M_{\rm a}$ and a dimensionless magnetization parameter, $Ψ\equiv B_*^2 R_*^2 (\dot M_{\rm a} v_{\rm esc})^{-1}$, where $\dot M_{\rm w}$ is the stellar wind mass outflow rate, $\dot M_{\rm a}$ the accretion rate, $B_*$ the stellar surface magnetic field strength, $R_*$ the stellar radius, and $v_{\rm esc}$ the surface escape speed. For parameters typical of accreting pre-main-sequence stars, this explains spin rates of $\sim 10$% of breakup speed for $\dot M_{\rm w} / \dot M_{\rm a} \sim 0.1$. Finally, the assumption that the stellar wind is driven by a fraction of the accretion power leads to an upper limit to the mass flow ratio of $\dot M_{\rm w} / \dot M_{\rm a} \la 0.6$.

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