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 p proporti onal to r(-p), with p = 1. The case of the singular isothermal sphere (p = 2) is not considered; however p = 1 has been shown observationall y to be a good representation of the density distribution in molecular cloud cores just before the beginning of collapse. The collapse is st udied with two independent numerical methods, a smoothed particle hydr odynamics (SPH) code with 200 000 particles and a finite difference co de with nested grids which give high spatial resolution in the inner r egions. Although previous numerical,studies have indicated that such a power-law distribution would not result in fragmentation into a binar y system, both codes show, in contrast, that multiple fragmentation do es occur in the central regions of the protostar. Thus the process of binary formation by fragmentation is shown to be consistent with the f act that a large fraction of young stars are observed to be in binary or multiple systems.