Collapse of a Molecular Cloud Core to Stellar Densities: The First Three-Dimensional Calculations

We present results from the first three-dimensional calculations ever to follow the collapse of a molecular cloud core (∼ 10 g cm) to stellar densities (> 0.01 g cm). The calculations resolve structures over 7 orders of magnitude in spatial extent (∼ 5000 AU − 0.1 R⊙), and over 17 orders of magnitude in density contrast. With these calculations, we consider whether fragmentation to form a close binary stellar system can occur during the second collapse phase. We find that, if the quasistatic core that forms before the second collapse phase is dynamically unstable to the growth of nonaxisymmetric perturbations, the angular momentum extracted from the central regions of the core, via gravitational torques, is sufficient to prevent fragmentation and the formation of a close binary during the subsequent second collapse. Subject headings: accretion – hydrodynamics – ISM: clouds – methods: numerical – stars: formation