临界旋转星衰减盘的磁旋转不稳定性及Be和Be/X射线盘的外部结构
Magnetorotational instability in decretion disks of critically rotating stars and the outer structure of Be and Be/X-ray disks
摘要：
快速旋转恒星的演化模型表明，恒星的自转速度可能接近临界速度。临界旋转的恒星不能更快地旋转，因此它们通过赤道外流盘失去多余的角动量。这种盘的径向延伸是未知的，部分原因是我们缺乏关于粘度径向变化的信息。我们研究了磁旋转不稳定性，这被认为是在外流盘异常粘度的起源。我们使用解析计算来研究浸没在磁场中的流出圆盘的稳定性。当等离子体参数足够大时，磁旋转不稳定性在靠近恒星的地方发展。在大半径处，在圆盘轨道速度大致等于声速的区域，不稳定性消失。磁旋转不稳定性是流出盘中反常粘性的一个合理来源。在圆盘径向速度接近声速的区域也是如此。因此，盘的音速半径可以粗略地认为是有效的外盘半径，尽管盘的物质可能会从恒星逃逸到星际介质中。角动量损失率的径向分布已经在那里得到，因此，可以用音速半径作为有效圆盘外半径来计算圆盘的质量损失率。本文讨论了用Be和Be/X射线双星观测确定外盘半径的可能性。
英文摘要：
Evolutionary models of fast-rotating stars show that the stellar rotational velocity may approach the critical speed. Critically rotating stars cannot spin up more, therefore they lose their excess angular momentum through an equatorial outflowing disk. The radial extension of such disks is unknown, partly because we lack information about the radial variations of the viscosity. We study the magnetorotational instability, which is considered to be the origin of anomalous viscosity in outflowing disks. We used analytic calculations to study the stability of outflowing disks submerged in the magnetic field. The magnetorotational instability develops close to the star if the plasma parameter is large enough. At large radii the instability disappears in the region where the disk orbital velocity is roughly equal to the sound speed. The magnetorotational instability is a plausible source of anomalous viscosity in outflowing disks. This is also true in the region where the disk radial velocity approaches the sound speed. The disk sonic radius can therefore be roughly considered as an effective outer disk radius, although disk material may escape from the star to the insterstellar medium. The radial profile of the angular momentum-loss rate already flattens there, consequently, the disk mass-loss rate can be calculated with the sonic radius as the effective disk outer radius. We discuss a possible observation determination of the outer disk radius by using Be and Be/X-ray binaries.
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