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外文期刊>The Astrophysical journal
>Three-dimensional Magnetohydrodynamic Modeling of Planetary Nebulae. II. The Formation of Bipolar and Elliptical Nebulae with Point-symmetric Structures and Collimated Outflows
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Three-dimensional Magnetohydrodynamic Modeling of Planetary Nebulae. II. The Formation of Bipolar and Elliptical Nebulae with Point-symmetric Structures and Collimated Outflows
This work presents three-dimensional, magnetohydrodynamic simulations of the formation and early evolution of a subgroup of planetary nebulae that exhibit a variety of point-symmetric structures. For bipolar nebulae, the formation of point-symmetric nebular shapes along the inner borders of their opposing lobes, and/or collimated outflows or jets internal or external to their cavities, is reproduced by considering a steady misalignment of the magnetic collimation axis with respect to the symmetry axis of the bipolar wind outflow, defined perpendicular to the equatorial density enhancement. Elliptical planetary nebulae with ansae displaced from the symmetry axis in point-symmetric fashion are reproduced through the same process by reducing the equatorial density enhancement. This mechanism represents an alternative explanation to some cases where morphological appearances give the impression of the action of a symmetric, rotating or precessing jet from the central source. The computational survey reveals that jet formation is detected only for dense enough winds with mass-loss rates 10-7 M☉ yr-1. For lower mass-loss rates the jets tend to vanish leaving behind only ansae-like structures at the tips of the lobes, as observed in some cases. The results are rather independent of the wind terminal velocity, since magnetized bubbles behave adiabatically for low wind velocities (100 km s-1), which in the absence of a magnetic field would behave as momentum driven.
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机译：这项工作提出了一个三维行星的磁流体动力学模拟，显示了具有各种点对称结构的行星状星云子群的形成和早期演化。对于双极星云，通过考虑其磁准直轴相对于磁准直轴的稳定不重合，可以沿其相对的瓣的内边界形成点对称的星状形状和/或在其腔内或外的准直的流出或射流。双极风流出的对称轴，垂直于赤道密度增强定义。通过减少赤道密度的增加，通过相同的过程复制了ansae以点对称方式偏离对称轴的椭圆形行星状星云。对于某些情况，其中形态学外观给人以来自中心源的对称，旋转或进动射流作用的印象，这种机制代表了另一种解释。计算调查表明，只有在质量损失率为10-7M☉yr-1的足够密集的风中才能检测到射流形成。对于较低的质量损失速率，喷射流倾向于消失，如在某些情况下所观察到的那样，仅在叶尖处留下了类似于ansae的结构。由于磁化气泡对于低风速（100 km s-1）绝热地表现，因此结果与风速终端速度无关，在没有磁场的情况下，磁化气泡会表现为动量驱动。
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