Self-Organization in Star Formation and Internal Structure of Molecular Clouds

恒星在分子云中活跃地形成，分子云包含分层的内部结构；它们包含细长的丝状云，包括球形云核，其中嵌入了双星或三星。这种分层结构是恒星在引力坍缩的作用下形成的。在这个项目中，我们把分子云在引力坍缩过程中的变形看作是自组织。为了理解引力坍缩的物理原理，我们进行了数值模拟。在我们的数值模拟中，考虑到自重力、碰撞场和旋转，我们跟踪了最初球形、丝状或盘状云的引力坍缩。虽然我们尝试了各种初始模型，但数值结果都是用以气体声速为特征的相似解来总结的。数值模拟的最终结果与云的初始质量、磁场和角动量关系不大。这表明，重力收缩云的结构是由重力和气体压力之间的平衡决定的。数值模拟和理论分析的主要结果如下：磁化云坍缩形成类似的圆盘，与初始磁场强度无关。以声速为参数的相似解可以很好地近似计算磁盘的密度和速度。2. 在引力作用下收缩的圆盘经常变形成棒状。在引力作用下收缩的球体通常会变形成圆盘或棒状。由于棒状恒星在破碎时不稳定，因此预计它会产生双星或三联体恒星。3. 用相似解很好地近似了引力坍缩的细丝。

Molecular clouds, in which stars form activcly, contain hierarchical internal structures ; they contain elongated filamentary clouds involving spherical cloud cores in which binaries or triplcts are embedded. This hicrarchical structure is produced when stars are formed by gravitational collapse. In this project we regard the deformation of molecular cloud during its gravitational collapse as self-organization. To understand the physics of gravitational collapse, we performed numerical simulations.We followed the gravitational collapse of initially either a spherical, filamentnry, or disky cloud in our numerical simulations taking account of self-gravity, inaguctic fields, and rotation. Although we tried various initial models, the numerical rcsults are summarized by similarity solutions characterized by the sound speed of gas. The final product in a numerical simulation depends little on the initial mass, magnetic field, and angular momentum of the cloud. This indicates that the structure of a gravitationally contracting cloud is determined by the balance between the gravity and gas pressure.The major findings from numerical simulations and theoretical analysis are as follows.1.A magnetized cloud collapses to form a similar disk irrespectively of its initial magnetic field strength. The density and velocity of the disk is well approximated by a similarity solution having the sound speed as a parameter. 2. A gravitationally contracting disk often deforms into a bar. A gravitationally contracting sphere often deforms into a disk or bar. Since a bar is unstable against fragmentation, it is expected to produce a binary or triplet of stars. 3. A gravitationally collapsing filament is well approximated by a similarity solution.