超吸积盘中的微物理耗散、turbulence和磁场
Microphysical dissipation, turbulence and magnetic fields in hyper-accreting discs
摘要：
超吸积盘出现在致密天体合并和碰撞中，并可能为伽马射线暴（GRB）提供动力。我们计算了这些圆盘中等离子体的微观粘度和电阻率，并讨论了它们的全球结构和演化的影响。在中微子冷却的最里面的区域，粘度主要由轻度简并电子提供，而电阻率由于相对论和简并度的影响而从斯皮策值修改。等离子体表现为几乎理想的MHD流体。在非理想MHD效应中，霍尔项相对来说是最重要的，而磁性普朗特数Pr（粘度与电阻率的比值）通常大于1：10 < Pr < 6000。外部辐射效率低的区域也显示高Pr。磁旋转不稳定性的数值模拟表明，饱和水平和角动量传输效率可能会大大提高在高Pr。如果这种行为持续存在一个强大的霍尔效应，我们预计，超吸积盘应强烈磁化和高度可变。不能在小尺度上消散的磁场的排出也可能有利于磁流出。我们注意到超吸积盘和X射线双星盘中的普朗特数之间的相似性，这表明伽玛暴和X射线双星吸积状态的后期活动之间的比较可能是富有成效的。我们的研究结果表明，高普朗特数MHD流的行为需要考虑在研究超吸积盘。
英文摘要：
Hyper-accreting discs occur in compact-object mergers and collapsars, and may power gamma-ray bursts (GRBs). We calculate the microscopic viscosity and resistivity of plasma in these discs, and discuss the implications for their global structure and evolution. In the neutrino-cooled innermost regions, the viscosity is provided mainly by mildly degenerate electrons, while the resistivity is modified from the Spitzer value due to the effects of both relativity and degeneracy. The plasma behaves as an almost ideal MHD fluid. Among the non-ideal MHD effects the Hall term is relatively the most important, while the magnetic Prandtl number, Pr (the ratio of viscosity to resistivity), is typically larger than unity: 10 < Pr < 6000. The outer radiatively inefficient regions also display high Pr. Numerical simulations of the magneto-rotational instability indicate that the saturation level and angular momentum transport efficiency may be greatly enhanced at high Pr. If this behaviour persists in the presence of a strong Hall effect we would expect that hyper-accreting discs should be strongly magnetised and highly variable. The expulsion of magnetic field that cannot be dissipated at small scales may also favour a magnetic outflow. We note the similaries between the Prandtl number in hyper-accreting discs and X-ray binary discs, which suggests that a comparison between late-time activity in GRBs and X-ray binary accretion states may be fruitful. Our results imply that the behavior of high Prandtl number MHD flows needs to be considered in studies of hyper-accreting discs.
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