模拟大质量恒星磁层的刚性场流体动力学方法
A Rigid-Field Hydrodynamics approach to modeling the magnetospheres of massive stars
摘要：
介绍了一种新的刚性场流体动力学方法来模拟大质量恒星在极强磁场极限下的磁层。将场线视为有效刚性，我们开发了描述沿每个场线的一维流动的流体动力学方程，受压力、辐射、重力和离心力的影响。我们数值求解这些方程的一个大的合奏的场线，建立一个三维的时间依赖模拟的模型星的参数类似的原型Bp星σ Ori E。由于沿每个场线的流动可以独立于其他场线求解，这种方法的计算成本是等效磁流体动力学处理的一小部分。模拟证实了许多以前的分析和数值研究的预测。来自相反磁半球的气流之间的碰撞导致强烈的冲击加热。激波后的等离子体最初通过X射线辐射冷却，并最终聚集成一个扭曲的，刚性旋转的盘，由有效（重力加离心）势的最小值的轨迹定义。但也出现了一些新的结果。对于远离恒星的磁力线，快速的区域发散增强了风的辐射加速，导致高激波速度（高达3，000 km/s）和硬X射线。此外，离心势能的释放继续加热激波后的风等离子体，温度高达激波本身温度的两倍左右。最后，在某些情况下，冷等离子体在积累盘可以振荡其平衡位置，可能是由于在相邻的激波后区域的辐射冷却不稳定性。
英文摘要：
We introduce a new Rigid-Field Hydrodynamics approach to modeling the magnetospheres of massive stars in the limit of very-strong magnetic fields. Treating the field lines as effectively rigid, we develop hydrodynamical equations describing the 1-dimensional flow along each, subject to pressure, radiative, gravitational, and centrifugal forces. We solve these equations numerically for a large ensemble of field lines, to build up a 3-dimensional time-dependent simulation of a model star with parameters similar to the archetypal Bp star sigma Ori E. Since the flow along each field line can be solved for independently of other field lines, the computational cost of this approach is a fraction of an equivalent magnetohydrodynamical treatment. The simulations confirm many of the predictions of previous analytical and numerical studies. Collisions between wind streams from opposing magnetic hemispheres lead to strong shock heating. The post-shock plasma cools initially via X-ray emission, and eventually accumulates into a warped, rigidly rotating disk defined by the locus of minima of the effective (gravitational plus centrifugal) potential. But a number of novel results also emerge. For field lines extending far from the star, the rapid area divergence enhances the radiative acceleration of the wind, resulting in high shock velocities (up to ~3,000 km/s) and hard X-rays. Moreover, the release of centrifugal potential energy continues to heat the wind plasma after the shocks, up to temperatures around twice those achieved at the shocks themselves. Finally, in some circumstances the cool plasma in the accumulating disk can oscillate about its equilibrium position, possibly due to radiative cooling instabilities in the adjacent post-shock regions.
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