A. Burkert et P. Bodenheimer, FRAGMENTATION IN A CENTRALLY CONDENSED PROTOSTAR, Monthly Notices of the Royal Astronomical Society, 280(4), 1996, pp. 1190-1200
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.