Numerical Study on Evolution from Magnetized Cloud to Protostar Using Three-Dimensional Nested Grid Method

利用三维磁流体力学（NM）模拟，研究了密度在102 ~ 1012cm3.1范围内星际响亮的收缩和恒星的形成。在嵌套网格方法中，使用了许多网格系统。细网格覆盖高密度中心，粗网格覆盖低密度包络，保证了自引力坍缩模拟的数值精度。开发了基于嵌套网格法的MHD仿真程序（二维和三维），其中采用针对嵌套网格改进的多网格迭代法求解自重力泊松方程。二维轴对称模拟表明，形成了一个垂直于磁场的圆盘，在中心形成密度为~ 10^10 cm-3的绝热核心后，由于核心向气流的角动量输运，气体垂直于圆盘喷出。三维MHD模拟发现了三种不同的，更多的绝热核心的演化，这将形成新生的恒星。(1)绝热岩心内发育螺旋密度波。臂的高密度（至少两个）部分收缩并形成双星原星（螺旋破碎）。(2)在等温圆盘中，非轴对称微扰生长，形成棒状结构。高密度的棒状绝热核心部分在自重力作用下坍缩，在棒状内部形成双星（棒状碎裂）。(3)绝热核心永远不会碎裂，中心尖峰核心继续收缩并形成单一的原恒星（不碎裂）。当角转速与磁通密度之比大于3倍10-7 y-1微G-1.5时，获得破碎条件。即使最初的旋转矢量与磁场不平行，在收缩过程中，磁场和旋转矢量在云的中心附近变得彼此对齐。圆盘沿与本地磁场垂直的方向运行。少

Using three-dimensional magnetohydrodynamical (NM) simulations, the contraction of the interstellar loud and star formation are studied for the density range from 102 to 1012 cm3.1. In the nested grid method, a number of grid systems are used. Fine grids cover the high-density center while coarse grids cover the low-density envelope, which guarantees the numerical accuracy in the simulation of self-gravitational collapse. MHD simulation programs (2-dimensional and 3-dimensional) based on the nested grid method was developed, in which the Poisson equation of the self gravity is solved with a multigrid iteration method modified for the nested grid.2. Axisymmetric 2D simulations show that a disk extending perpendicular to the magnetic field is made and that after the adiabatic core forms at the center with a density of〜10^10 cm-3 gas is ejected perpendicular to the disk owing to the angular momentum transport from the core to the flow.3. Three-dimensional MHD simulations found three diffe … More rent evolutions for the adiabatic core which will form new-born stars.(1) Spiral density wave develops in the adiabatic core. High-density (at least two) parts of the arms contract and form binary protostars (spiral fragmentation).(2) In the isothermal disk, nonaxisymmetric perturbations grow and form bar structure. High-density parts of the bar-shaped adiabatic core collapse by the self-gravity and binary protostars form in the bar (bar fragmentation).(3) The adiabatic core never fragment and a centrally-peaked core continues to contract and form a single protostar (nofragmentation).4. Condition for fragmentation is obtained as the ratio of the angular rotation speed to the magnetic flux density is larger than 3 times 10-7 y-1 micro G-1.5. Even if the initial rotation vector is not parallel to the magnetic field, magnetic field and rotation vector becomes to be aligned each other near the center of the cloud in the course of contraction. A disk runs in the perpendicular direction to the local magnetic field. Less