Collapse and fragmentation of rotating magnetized clouds — II. Binary formation and fragmentation of first cores
Collapse and fragmentation of rotating magnetized clouds — II. Binary formation and fragmentation of first cores
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DOI：
10.1111/j.1365-2966.2005.09327.x
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发表时间：
2005-06
影响因子：
4.8
通讯作者：
M. Machida;Tomoaki Matsumoto;T. Hanawa;K. Tomisaka
中科院分区：
文献类型：
--
作者：
M. Machida;Tomoaki Matsumoto;T. Hanawa;K. Tomisaka
继论文I之后，本文用三维磁流体动力学嵌套网格模拟方法研究了旋转磁化云的演化和碎裂。在等温逃逸坍缩之后，绝热气体在云的中心形成了原恒星的第一核。当等温气体在一个收缩的圆盘中稳定碎裂时，绝热核心常常会碎裂成几个碎片。研究了碎裂和二进制形成的条件。所有显示碎裂的岩心在几何上都很薄，因为直径与厚度之比大于3。发现两种碎裂模式：（1）当薄盘受到离心力的支撑时，盘碎裂成环形结构（环形碎裂）。这在快速旋转的绝热核心中实现为β> 0.2τ −1，其中β和τ ff分别表示核心的角旋转速度和自由落体时间。(2)另一方面，在等温阶段，对于强磁化或快速旋转的云，盘变形为细长的棒。条断裂成2 - 4个片段（条断裂）。即使圆盘很薄，由磁力或热压控制的圆盘也是稳定的，并形成单一的紧凑体。在环形或条形破碎模式中，碎片收缩，并且一对流出物从致密核心附近喷出。在环碎裂过程中，轨道角动量大于自旋角动量。另一方面，在棒碎裂中，碎片通常很快合并，因为在这种情况下，轨道角动量小于自旋角动量。还显示了与观察的比较。
Subsequent to Paper I, the evolution and fragmentation of a rotating magnetized cloud are studied with use of three-dimensional magnetohydrodynamic nested grid simulations. After the isothermal runaway collapse, an adiabatic gas forms a protostellar first core at the centre of the cloud. When the isothermal gas is stable for fragmentation in a contracting disc, the adiabatic core often breaks into several fragments. Conditions for fragmentation and binary formation are studied. All the cores which show fragmentation are geometrically thin, as the diameter-to-thickness ratio is larger than 3. Two patterns of fragmentation are found. (1) When a thin disc is supported by centrifugal force, the disc fragments into a ring configuration (ring fragmentation). This is realized in a rapidly rotating adiabatic core as �> 0.2τ −1 , where � and τ ff represent the angular rotation speed and the free-fall time of the core, respectively. (2) On the other hand, the disc is deformed to an elongated bar in the isothermal stage for a strongly magnetized or rapidly rotating cloud. The bar breaks into 2‐4 fragments (bar fragmentation). Even if a disc is thin, the disc dominated by the magnetic force or thermal pressure is stable and forms a single compact body. In either ring or bar fragmentation mode, the fragments contract and a pair of outflows is ejected from the vicinities of the compact cores. The orbital angular momentum is larger than the spin angular momentum in the ring fragmentation. On the other hand, fragments often quickly merge in the bar fragmentation, since the orbital angular momentum is smaller than the spin angular momentum in this case. Comparison with observations is also shown.