Fragmentation in a centrally condensed protostar⋆

Hydrodynamical calculations in three space dimensions of the collapse of an isothermal, centrally condensed, rotating 1 M protostellar cloud are presented.A numerical algorithm involving nested subgrids is used to resolve the region where fragmentation occurs in the central part of the protostar.A previous calculation by Boss, which produced a hierarchical multiple system, is evolved further, at comparable numerical resolution, and the end result is a binary, with more than half of the mass of the original cloud, whose orbital separation increases with time as a result of accretion of high-angular momentum material and as a result of merging with fragments that have formed farther out.Repeating the calculation with signi cantly higher resolution, we nd that a sequence of binaries can be induced by fragmentation of circumbinary disks.The stability of the resulting multiple system is investigated using n-body calculations, which indicate that such a system would transform on a short time scale into a more stable hierarchical structure.The outermost and most massive binary which forms in the high-resolution run has properties similar to that of the binary found in the low-resolution calculation.Thus the basic outcome is shown to be independent of the numerical spatial resolution.The high-resolution run, in addition, leads to the formation of a system of smaller fragments, which might be important for the understanding of the origin of close binaries with low-mass components and of low mass single stars.