Planetary accumulation with a continuous supply of planetesimals

Previous calculations of the accumulation of small (similar to 10 km) plane tesimals at similar to 1 AU to form Mars-sized bodies assumed that the init ial assemblage of planetesimals were all present at the outset. This is an obviously reasonable assumption in systems in which the time scale for grow th time of similar to 10(26) g planetary bodies is long compared to estimat es of the evolutionary time scale of a protosolar disk, as was the case in the pioneering work of Safronov (1969). It is now found that as a result of the preplanetary assemblage being unstable with respect to the runaway gro wth of the largest bodies, this is unlikely to be the case. The more realis tic alternative of adding the initial planetesimals on a similar to 10(5) y ear time scale is considered here, as well as the consequences of the initi al planetesimals being considerably smaller than those assumed previously. It is found that although the time scale for runaway growth is now actually controlled by the availability of planetesimals, for planetesimal producti on time scales of similar to 10(5) yrs, the final consequences are very sim ilar. These calculations do show, however, that as a consequence of continu ous infall during the runaway growth process, the late initial planetesimal s are likely to be catastrophically disrupted by mutual collisions. For thi s reason, a more detailed treatment of the growth of planetesimals into pla netary embryos will require a better understanding of the difficult problem of formation of the initial planetesimals themselves.