The Fragmentation of Cores and the Initial Binary Population

Almost all young stars are found in multiple systems. This suggests that protostellar cores almost always fragment into multiple objects. The observed properties of multiple systems such as their separation distribution and mass ratios provide strong constraints on star formation theories. We review the observed properties of young and old multiple systems and find that the multiplicity of stars changes. Such an evolution is probably due to (a) the dynamical decay of small-N systems and/or (b) the destruction of multiple systems within dense clusters. We review simulations of the fragmentation of rotating and turbulent molecular cores. Such models almost always produce multiple systems, however the properties of those systems do not match observations at all well. Magnetic fields appear to supress fragmentation, prehaps suggesting that they are not dynamically important in the formation of multiple systems. We finish by discussing possible reasons why theory fails to match observation, and the future prospects for this field.