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

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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.