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arXiv 1301.4543astro-ph.HE
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薄吸积盘的大尺度磁场

The large scale magnetic fields of thin accretion disks

Xinwu Cao, H. C. Spruit

摘要:

穿过吸积盘的大尺度磁场是喷流形成模型中的一个重要组成部分。对于这样一个大尺度场的起源,最吸引人的设想是来自星际介质或伴星的吸积盘中的气体对流场。然而,它被认识到,向外扩散的吸积场是快速的向内吸积速度相比,在一个几何薄的吸积盘,如果普朗特数Pm的值是约1。在这项工作中,我们重新审视这个问题,考虑到磁盘的角动量主要是由磁驱动的外流。盘的径向速度由于流出的存在而显著增加。使用一个简化的模型的垂直盘结构,我们发现,即使是适度弱的领域可以通过磁风平衡向外扩散造成足够的角动量损失。有两个平衡点,一个在低场强对应于等离子体β在中平面的订单几百,和一个强吸积场,β ~1。我们推测,第一个是相关的吸积弱,可能是外部,通过磁盘的外部部分的字段,而后者可以解释的趋势,观察到在全三维数值模拟,强大的通量束在磁盘的中心,以保持限制,尽管强大的MRI湍流周围。

英文摘要:

Large scale magnetic field threading an accretion disk is a key ingredient in the jet formation model. The most attractive scenario for the origin of such a large scale field is the advection of the field by the gas in the accretion disk from the interstellar medium or a companion star. However, it is realized that outward diffusion of the accreted field is fast compared to the inward accretion velocity in a geometrically thin accretion disk if the value of the Prandtl number Pm is around unity. In this work, we revisit this problem considering the angular momentum of the disk is removed predominantly by the magnetically driven outflows. The radial velocity of the disk is significantly increased due to the presence of the outflows. Using a simplified model for the vertical disk structure, we find that even moderately weak fields can cause sufficient angular momentum loss via a magnetic wind to balance outward diffusion. There are two equilibrium points, one at low field strengths corresponding to a plasma-beta at the midplane of order several hundred, and one for strong accreted fields, beta~1. We surmise that the first is relevant for the accretion of weak, possibly external, fields through the outer parts of the disk, while the latter one could explain the tendency, observed in full 3D numerical simulations, of strong flux bundles at the centers of disk to stay confined in spite of strong MRI turbulence surrounding them.

机构、总结、中文摘要均由AI分析而来,可能存在错误,请以论文正文为准。

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