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102-1012 cm-3.1范围内星际噪声的收缩和恒星的形成。在嵌套网格法中，使用了多个网格系统。细网格覆盖高密度中心，粗网格覆盖低密度包络，保证了自引力塌陷模拟的数值精度。编制了基于嵌套网格法的MHD模拟程序(二维和三维)，采用改进的多重网格迭代法求解自重泊松方程。轴对称二维模拟结果表明，在中心形成密度约为10~10 cm-3的绝热核后，由于角动量的输运，气体垂直于圆盘射出，形成垂直于磁场的圆盘。三维磁流体模拟发现三种不同的…绝热核有更多的租金演化，这将形成新生的恒星。(1)在绝热核中发展了螺旋密度波。臂的高密度部分(至少两个)收缩并形成双星(螺旋碎裂)。(2)在等温盘中，非轴对称微扰增长并形成棒状结构。棒状绝热核的高密度部分在自引力的作用下坍塌，双星在棒中形成(棒状碎裂)。(3)绝热核从不碎裂，中心峰状的核继续收缩，形成单一的原星(无碎裂)。得出了角转速与磁通密度之比大于3倍10~(-7)y~(-1)·m~(-1)·g~(-1)时的破碎条件。即使初始旋转矢量与磁场不平行，磁场和旋转矢量在收缩过程中也会在云中心附近彼此对准。圆盘沿垂直于局部磁场的方向运行。较少

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