Gravitational Collapse of Magnetized Clouds. I. Ideal Magnetohydrodynamic Accretion Flow
Gravitational Collapse of Magnetized Clouds. I. Ideal Magnetohydrodynamic Accretion Flow
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DOI：
10.1086/505257
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发表时间：
2006-04
期刊：
影响因子：
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通讯作者：
Daniele Galli;S. Lizano;F. Shu;A. Allen
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文献类型：
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作者：
Daniele Galli;S. Lizano;F. Shu;A. Allen
本文研究了理想磁流体力学（MHD）体系中等温磁化旋转云的自相似坍缩。在距离吸积原恒星很小的范围内，我们找到了一个解析解，它对应于自由落体到一个中心质点上。密度分布不是球对称的，而是取决于磁力线的质量负载，这可以通过将我们的内部解与艾伦等人先前计算的外部坍缩解相匹配来获得。场冻结下中心质量点捕获的磁场浓度，与起始状态的细节无关，产生了分裂的磁场配置，其中磁场强度随着距中心的距离的平方反比而增加。在这样的条件下，流入最终变成亚阿尔夫维尼克，并且通过磁制动向外传递角动量非常有效，从而防止形成离心支撑盘。相反，下落气体的方位角速度在中心处减小到零，气体螺旋进入星星。因此，重要的动态磁场水平的消散是年轻恒星周围原行星盘形成的基本条件。
We study the self-similar collapse of an isothermal magnetized rotating cloud in the ideal magnetohydrodynamic (MHD) regime. In the limit of small distance from the accreting protostar, we find an analytic solution that corresponds to free fall onto a central mass point. The density distribution is not spherically symmetric but depends on the mass loading of magnetic field lines, which can be obtained by matching our inner solution to an outer collapse solution previously computed by Allen et al. The concentration of magnetic field trapped by the central mass point under field freezing, independent on the details of the starting state, creates a split-monopole configuration in which the magnetic field strength increases as the inverse square of the distance from the center. Under such conditions, the inflow eventually becomes sub-Alfvènic and the outward transfer of angular momentum by magnetic braking very efficient, thus preventing the formation of a centrifugally supported disk. Instead, the azimuthal velocity of the infalling gas decreases to zero at the center, and the gas spirals into the star. Therefore, the dissipation of dynamically important levels of magnetic field is a fundamental requisite for the formation of protoplanetary disks around young stars.