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arXiv 0801.0436astro-ph
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吸积动力恒星风II:恒星风扭矩的数值解

Accretion-Powered Stellar Winds II: Numerical Solutions for Stellar Wind Torques

Sean Matt, Ralph E. Pudritz

摘要:

为了解释在大部分吸积前主序星(CTTS)中观察到的缓慢旋转,我们探索了恒星风在扭转恒星中的作用。为了使这种机制有效,恒星风需要有相对较高的流出率,因此可能由吸积过程本身提供动力。在这里,我们使用数值磁流体动力学模拟计算详细的2维(轴对称)恒星风的解决方案,以确定对恒星的自旋扭矩。我们探索了一系列与CTTS相关的参数,包括恒星质量、半径、自转率、表面磁场强度、质量损失率和风加速率的变化。我们还考虑了偶极和四极磁场的几何形状。我们的模拟表明,恒星风扭矩是足够大的旋转下来的“典型的”CTTS是重要的,质量损失率为$\sim 10^{-9} M_\odot$年$^{-1}$。风是广角,自准直流,如预期的磁转子风与适度快速旋转。四极场的情况下产生的扭矩比具有相同的表面场强度的偶极子弱得多,这表明磁几何形状在确定扭矩中起着根本性的作用。具有变化的风加速率的情况下,扭矩的变化要小得多,这表明风驱动的细节不太重要。我们使用我们的计算结果来拟合风中有效Alfvén半径以及扭矩的半解析公式。这允许相当大的预测能力,并且是对现有近似的改进。

英文摘要:

[Abridged] In order to explain the slow rotation observed in a large fraction of accreting pre-main-sequence stars (CTTSs), we explore the role of stellar winds in torquing down the stars. For this mechanism to be effective, the stellar winds need to have relatively high outflow rates, and thus would likely be powered by the accretion process itself. Here, we use numerical magnetohydrodynamical simulations to compute detailed 2-dimensional (axisymmetric) stellar wind solutions, in order to determine the spin down torque on the star. We explore a range of parameters relevant for CTTSs, including variations in the stellar mass, radius, spin rate, surface magnetic field strength, the mass loss rate, and wind acceleration rate. We also consider both dipole and quadrupole magnetic field geometries. Our simulations indicate that the stellar wind torque is of sufficient magnitude to be important for spinning down a ``typical'' CTTS, for a mass loss rate of $\sim 10^{-9} M_\odot$ yr$^{-1}$. The winds are wide-angle, self-collimated flows, as expected of magnetic rotator winds with moderately fast rotation. The cases with quadrupolar field produce a much weaker torque than for a dipole with the same surface field strength, demonstrating that magnetic geometry plays a fundamental role in determining the torque. Cases with varying wind acceleration rate show much smaller variations in the torque suggesting that the details of the wind driving are less important. We use our computed results to fit a semi-analytic formula for the effective Alfvén radius in the wind, as well as the torque. This allows for considerable predictive power, and is an improvement over existing approximations.

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