FORMATION OF HIERARCHICAL MULTIPLE PROTOSTELLAR CORES

BINARY pre-main-sequence stars 1,2 seem to occur as frequently as binary main-sequence stars 3,4; triple pre-main-sequence star systems have also been detected 5, and hierarchical main-sequence multiple stars continue to be identified 6. (Hierarchical systems contain both closely spaced stars and stars orbiting at much greater distances.) These observations suggest that essentially all binary stars were formed before the main-sequence phase of evolution. The detection of a number of binary young stellar objects 7,8 seems to indicate that binary formation must occur no later than the protostellar phase (further observations are needed to establish if this is the case for multiple star systems). Here I describe numerical hydrodynamical calculations showing that stable hierarchical systems of multiple protostellar cores can form through gravitationally driven fragmentation during the collapse of an isolated gas cloud, suggesting that the hierarchical systems observed may be the result of the hydrodynamical collapse of rapidly rotating clouds.