Multiple fragmentation in collapsing protostars*

Abstract

Hydrodynamical calculations in three space dimensions of the collapse of an isothermal, rotating $$1-{\rm M}_{\odot}$$ protostellar cloud are presented. A numerical algorithm involving nested subgrids is used to resolve the region of initial binary formation in the central part of the protostar. An extensive series of numerical tests confirms the accuracy of the computer program. Comparisons are given with a standard test case of binary fragmentation in a collapsing isothermal cloud of initially uniform density. In a simulation with a 50 per cent initial perturbation a binary is clearly formed. In another simulation with a 10 per cent initial perturbation, however, calculations show the initial formation of a binary and a connecting bar, followed by fragmentation of the bar. The binary components are surrounded by accretion discs of radius ~ 100 au. This result is shown to be independent of the grid spacing employed. It is suggested that the further evolution of this system is likely to produce some isolated low-mass fragments. In both the above cases (i.e. 50 and 10 per cent perturbations) a detailed analysis of the momentum of the forming fragments indicates that they are initially on a highly eccentric orbit. The subsequent interaction of each fragment with the other's accretion disc could result in circularization and the formation of a shorter period multiple system.