Tomisaka K
Astrophys J. 2000 Jan 1;528(1):L41-L44. doi: 10.1086/312417.
If the angular momentum of the molecular cloud core were conserved during the star formation process, a newborn star would rotate much faster than its fission speed. This constitutes the angular momentum problem of newborn stars. In this Letter, the angular momentum transfer in the contraction of a rotating magnetized cloud is studied with axisymmetric MHD simulations. Because of the large dynamic range covered by the nested-grid method, the structure of the cloud in the range from 10 AU to 0.1 pc is explored. First, the cloud experiences a runaway collapse, and a disk forms perpendicularly to the magnetic field, in which the central density increases greatly in a finite timescale. In this phase, the specific angular momentum j of the disk decreases to about one-third of the initial cloud. After the central density of the disk exceeds approximately 1010 cm-3, the infall on to the central object develops. In this accretion stage, the rotation motion and thus the toroidal magnetic field drive the outflow. The angular momentum of the central object is transferred efficiently by the outflow as well as by the effect of the magnetic stress. In 7000 yr from the core formation, the specific angular momentum of the central 0.17 M middle dot in circle decreases a factor of 10-4 from the initial value (i.e., from 1020 to 1016 cm2 s-1).
如果分子云核的角动量在恒星形成过程中得以守恒，那么一颗新生恒星的旋转速度将比其裂变速度快得多。这就构成了新生恒星的角动量问题。在本快报中，利用轴对称磁流体动力学模拟研究了旋转磁化云收缩过程中的角动量转移。由于嵌套网格方法覆盖的动态范围很大，因此对范围从10天文单位到0.1秒差距的云结构进行了探索。首先，云经历失控坍缩，并形成一个垂直于磁场的盘，其中中心密度在有限的时间尺度内大幅增加。在这个阶段，盘的比角动量j减小到初始云的约三分之一。当盘的中心密度超过约10¹⁰厘米⁻³后，向中心天体的吸积开始发展。在这个吸积阶段，旋转运动以及环形磁场驱动外流。中心天体的角动量通过外流以及磁应力的作用而有效地转移。从核心形成起7000年后，中心0.17 M⊙的比角动量从初始值（即从10²⁰厘米²秒⁻¹）减小了10⁻⁴倍。
