Protostellar Fragmentation in a Power-law Density Distribution *

Hydrodynamical calculations in three space dimensions of the collapse of an isothermal, rotating 1 M⊙ protostellar cloud are presented. The initial density stratification is a power law with density ρ ∝ r, with p = 1. The case of the singular isothermal sphere (p = 2) is not considered; however p = 1 has been shown observationally to be a good representation of the density distribution in molecular cloud cores just before the beginning of collapse. The collapse is studied with two independent numerical methods, an SPH code with 200,000 particles, and a finite-difference code with nested grids which give high spatial resolution in the inner regions. Although previous numerical studies have indicated that such a power-law distribution would not result in fragmentation into a binary system, both codes show, in contrast, that multiple fragmentation does occur in the central regions of the protostar. Thus the process of binary formation by fragmentation is shown to be consistent with the fact that a large fraction of young stars are observed to be in binary or multiple systems.