PROTOSTELLAR ACCRETION FLOWS DESTABILIZED BY MAGNETIC FLUX REDISTRIBUTION
PROTOSTELLAR ACCRETION FLOWS DESTABILIZED BY MAGNETIC FLUX REDISTRIBUTION
复制标题
原恒星吸积流因磁通量重新分布而不稳定
DOI：
10.1088/0004-637x/757/1/77
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发表时间：
2012
期刊：
影响因子：
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通讯作者：
Charlottesville
中科院分区：
文献类型：
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作者：
R. Krasnopolsky;Zhi;H. Shang;B. Z. A. Sinica;Taipei;U. Virginia;Charlottesville
磁通量的重新分布是星星在分子云的致密核心中形成的问题的核心，这些分子云被磁化到一个现实的水平。如果一个典型的核心的所有磁通量都被拖进中央的星星，那么恒星的磁场强度将比观测值高出几个数量级。这个众所周知的磁通量问题原则上可以通过非理想MHD效应来解决。二维（轴对称）计算表明，双极扩散，特别是，可以传输磁通量相对于物质向外，使材料进入中心的对象，而不会拖动场线沿着。我们通过模拟表明，这种轴对称的原恒星吸积流是不稳定的三维磁交换不稳定的方位角方向。不稳定性是由进入中心物体的物质重新分配的磁通量驱动的。它通常在从星星形成的星前阶段到原恒星质量吸积阶段的过渡期间开始发展。在后一阶段，磁通量主要通过强磁化的低密度区域的平流向外输送，这些区域对坍缩的流入进行扩张。引力驱动的坍缩和磁力驱动的膨胀之间的争斗导致了一个比以前设想的更无序的高度非连续的内部吸积流。通过平流的磁通量的有效向外传输降低了小半径处的场强，使得磁制动效率较低，并且原则上更容易形成旋转支撑的盘。然而，我们在任何旋转坍缩模拟中都没有发现这种盘的证据。我们的结论是，内部的原恒星吸积流的形状在很大程度上由磁通再分布驱动的磁交换不稳定性。在这样的环境中，磁盘是如何形成的还不清楚。
Magnetic flux redistribution lies at the heart of the problem of star formation in dense cores of molecular clouds that are magnetized to a realistic level. If all of the magnetic flux of a typical core were to be dragged into the central star, the stellar field strength would be orders of magnitude higher than the observed values. This well-known magnetic flux problem can in principle be resolved through non-ideal MHD effects. Two-dimensional (axisymmetric) calculations have shown that ambipolar diffusion, in particular, can transport magnetic flux outward relative to matter, allowing material to enter the central object without dragging the field lines along. We show through simulations that such axisymmetric protostellar accretion flows are unstable in three dimensions to magnetic interchange instability in the azimuthal direction. The instability is driven by the magnetic flux redistributed from the matter that enters the central object. It typically starts to develop during the transition from the prestellar phase of star formation to the protostellar mass accretion phase. In the latter phase, the magnetic flux is transported outward mainly through advection by strongly magnetized low-density regions that expand against the collapsing inflow. The tussle between the gravity-driven infall and magnetically driven expansion leads to a highly filamentary inner accretion flow that is more disordered than previously envisioned. The efficient outward transport of magnetic flux by advection lowers the field strength at small radii, making the magnetic braking less efficient and the formation of rotationally supported disks easier in principle. However, we find no evidence for such disks in any of our rotating collapse simulations. We conclude that the inner protostellar accretion flow is shaped to a large extent by the flux redistribution-driven magnetic interchange instability. How disks form in such an environment is unclear.