FRAGMENTATION IN A CENTRALLY CONDENSED PROTOSTAR

Hydrodynamical calculations in three space dimensions of the collapse of an isothermal, centrally condensed, rotating 1-M(.) protostellar cl oud are presented. A numerical algorithm involving nested subgrids is used to resolve the region where fragmentation occurs in the central p art of the protostar. A previous calculation by Boss, which produced a hierarchical multiple system, is evolved further, at comparable numer ical resolution, and the end result is a binary, with more than half o f the mass of the original cloud, plus a central object formed from th e merger of small inner fragments. The orbital separation of the main binary increases with time as a result of accretion of high,angular-mo mentum material, and asa result of merging with fragments that have fo rmed farther out. Repeating the calculation with significantly higher resolution, we find that a sequence of binaries can be induced by frag mentation of circumbinary discs. The stability of the resulting multip le system is investigated using n-body calculations, which indicate th at such a system would transform on a short time-scale into a more sta ble hierarchical structure. The outermost and most massive binary whic h forms in the high-resolution run has properties similar to that of t he binary found in the low-resolution calculation. Thus the basic outc ome is shown to be independent of the numerical spatial resolution. Th e high-resolution run, in addition, leads to the formation of an inner system of smaller fragments, which might be important fur the underst anding of the origin of close binaries with low-mass components and of low-mass single stars.