A Three-dimensional Simulation of a Magnetized Accretion Disk: Fast Funnel Accretion onto a Weakly Magnetized Star
A Three-dimensional Simulation of a Magnetized Accretion Disk: Fast Funnel Accretion onto a Weakly Magnetized Star
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DOI：
10.3847/1538-4357/aab5b3
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发表时间：
2018-01
期刊：
影响因子：
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通讯作者：
S. Takasao;K. Tomida;K. Iwasaki;T. Suzuki
中科院分区：
文献类型：
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作者：
S. Takasao;K. Tomida;K. Iwasaki;T. Suzuki
我们提出了一个全球性的，三维磁流体动力学模拟的吸积盘旋转，弱磁化中心星星的结果。圆盘被一个弱的、大尺度的极向磁场穿过，中心的星星最初没有强的恒星磁层。我们的模拟研究了从湍流吸积盘到星星的吸积流的结构。模拟显示，即使没有恒星磁层，高纬度地区的星星也会发生快速吸积。我们发现，失败的磁盘风成为快速，高纬度吸积作为一个结果的角动量交换介导的磁场远远高于磁盘，其中的洛伦兹力，减速的旋转运动的气体可以比较的离心力。与经典的磁层吸积场景不同，快速吸积流不受恒星磁层磁场的引导。然而，吸积速度达到自由落体速度在恒星表面由于有效的角动量损失在一个遥远的地方从星星。这项研究提供了一个可能的解释，为什么磁场一般不足以形成磁层的Herbig Ae/Be恒星也显示出快速吸积的迹象。在我们的模型中，磁驱动射流不是从盘形成的。由于帕克不稳定性干扰了磁场的放大作用，差动旋转不能产生足够强的磁场来加速喷流。
We present the results of a global, three-dimensional magnetohydrodynamics simulation of an accretion disk with a rotating, weakly magnetized central star. The disk is threaded by a weak, large-scale poloidal magnetic field, and the central star has no strong stellar magnetosphere initially. Our simulation investigates the structure of the accretion flows from a turbulent accretion disk onto the star. The simulation reveals that fast accretion onto the star at high latitudes occurs even without a stellar magnetosphere. We find that the failed disk wind becomes the fast, high-latitude accretion as a result of angular momentum exchange mediated by magnetic fields well above the disk, where the Lorentz force that decelerates the rotational motion of gas can be comparable to the centrifugal force. Unlike the classical magnetospheric accretion scenario, fast accretion streams are not guided by magnetic fields of the stellar magnetosphere. Nevertheless, the accretion velocity reaches the free-fall velocity at the stellar surface due to the efficient angular momentum loss at a distant place from the star. This study provides a possible explanation why Herbig Ae/Be stars whose magnetic fields are generally not strong enough to form magnetospheres also show indications of fast accretion. A magnetically driven jet is not formed from the disk in our model. The differential rotation cannot generate sufficiently strong magnetic fields for the jet acceleration because the Parker instability interrupts the field amplification.