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The fragmentation of a molecular cloud during collapse into several protostars is a promising method for producing the observed frequency and characteristics of binary and multiple star systems. Fragmentation to form a binary or multiple system occurs during the collapse of a molecular cloud either due to an initial density perturbation (e.g. a non-spherical cloud) or through the fragmentation of a protostellar disc.
Thus far hydrodynamical simulations can not follow fragmentation calculations significantly beyond the initial formation of the protostars, as the calculation time becomes too long. At this stage, most of the material from the initial cloud still has to be accreted on to the system. Hence, although the formation of binary (or multiple) protostars can occur, no conclusions about the final state of the resulting stellar system can be made.
We present a method for following fragmentation calculations until the majority of the gas has been accreted by the protostars. The high density protostellar regions in Smoothed Particle Hydrodynamics simulations are replaced by accreting, non-gaseous particles with appropriate boundary conditions. This allows us to follow the dynamics of the accretion of the cloud material on to a protobinary system, and so obtain its final state. Results from these calculations can then be compared to observations.
