MULTIPLE FRAGMENTATION IN COLLAPSING PROTOSTARS

Hydrodynamical calculations in three space dimensions of the collapse of an isothermal, rotating 1-M. protostellar cloud are presented. A nu merical algorithm involving nested subgrids is used to resolve the reg ion of initial binary formation in the central part of the protostar. An extensive series of numerical tests confirms the accuracy of the co mputer program. Comparisons are given with a standard test case of bin ary fragmentation in a collapsing isothermal cloud of initially unifor m density. In a simulation with a 50 per cent initial perturbation a b inary is clearly formed. In another simulation with a 10 per cent init ial perturbation, however, calculations show the initial formation of a binary and a connecting bar, followed by fragmentation of the bar. T he binary components are surrounded by accretion discs of radius appro ximately 100 au. This result is shown to be independent of the grid sp acing employed. It is suggested that the further evolution of this sys tem is likely to produce some isolated low-mass fragments. In both the above cases (i.e. 50 and 10 per cent perturbations) a detailed analys is of the momentum of the forming fragments indicates that they are in itially on a highly eccentric orbit. The subsequent interaction of eac h fragment with the other's accretion disc could result in circulariza tion and the formation of a shorter period multiple system.