arXiv 1301.65452013-01-25astro-ph.SR

磁场-旋转失调能解决原恒星盘形成中的磁制动灾难吗？

Does Magnetic Field-Rotation Misalignment Solve the Magnetic Braking Catastrophe in Protostellar Disk Formation?

Zhi-Yun Li, Ruben Krasnopolsky, Hsien Shang

备注：: 29 pages, 8 figures, submitted to ApJ
DOI：: 10.1088/0004-637X/774/1/82

摘要：

恒星形成于分子云的致密核心中，这些分子云被观察到明显的磁化。在最简单的情况下，一个层流（非湍流）的核心与旋转轴对齐的磁场，无论是分析的考虑和数值模拟表明，形成一个大的，$10^2\au$规模，旋转支持原恒星盘抑制磁制动在理想的磁流体动力学极限为一个现实水平的核心磁化。这种形成原恒星盘的理论困难被称为“磁制动灾难”。\citeauthor{HennebelleCiardi2009}和\citeauthor{Joos+2012}提出的一个可能的解决方案是，磁场和旋转轴之间的不对准可能会削弱磁制动，足以使磁盘形成。我们通过数值模拟定量评估这种可能性。我们证实了\citeauthor{Joos+2012}的基本结果，即错位确实有利于盘的形成。在具有无量纲质量-通量比$\gtrsim 5$的相对弱磁化的磁芯中，它使得能够形成旋转支撑盘，否则如果磁场和旋转轴对齐，则会抑制旋转支撑盘。然而，对于更强磁化的芯，即使对于$90\degree$的最大倾斜角，盘形成仍然被抑制。如果致密的核心被强烈磁化，如OH塞曼观测所示（具有平均无量纲质量-通量比$\sim 2$），那么仅仅是不对准就很难在大多数核心中形成磁盘。我们的结论是，虽然有利于磁盘的形成，特别是对于相对较弱的磁场的情况下，错位并不能完全解决灾难性的磁制动的问题。

英文摘要：

Stars form in dense cores of molecular clouds that are observed to be significantly magnetized. In the simplest case of a laminar (non-turbulent) core with the magnetic field aligned with the rotation axis, both analytic considerations and numerical simulations have shown that the formation of a large, $10^2\au$-scale, rotationally supported protostellar disk is suppressed by magnetic braking in the ideal MHD limit for a realistic level of core magnetization. This theoretical difficulty in forming protostellar disks is termed "magnetic braking catastrophe". A possible resolution to this problem, proposed by \citeauthor{HennebelleCiardi2009} and \citeauthor{Joos+2012}, is that misalignment between the magnetic field and rotation axis may weaken the magnetic braking enough to enable disk formation. We evaluate this possibility quantitatively through numerical simulations. We confirm the basic result of \citeauthor{Joos+2012} that the misalignment is indeed conducive to disk formation. In relatively weakly magnetized cores with dimensionless mass-to-flux ratio $\gtrsim 5$, it enabled the formation of rotationally supported disks that would otherwise be suppressed if the magnetic field and rotation axis are aligned. For more strongly magnetized cores, disk formation remains suppressed, however, even for the maximum tilt angle of $90\degree$. If dense cores are as strongly magnetized as indicated by OH Zeeman observations (with a mean dimensionless mass-to-flux ratio $\sim 2$), it would be difficult for the misalignment alone to enable disk formation in the majority of them. We conclude that, while beneficial to disk formation, especially for the relatively weak field case, the misalignment does not completely solve the problem of catastrophic magnetic braking in general.

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