On the Initial Mass Function of Population III Stars

The collapse and fragmentation of filamentary primordial gas clouds are explored using 1D and 2D hydrodynamical simulations coupled with the nonequilibrium processes of H2 formation. The simulations show that depending upon the initial density, there are two occasions for the fragmentation of primordial filaments. If a filament has relatively low initial density, the radial contraction is slow due to less effective H2 cooling. This filament tends to fragment into dense clumps before the central density reaches 10^8-9 cm^-3, where H2 cooling by three-body reactions is effective and the fragment mass is more massive than some tens M_\. In contrast, if a filament is initially dense, the more effective H2 cooling with the help of three-body reactions allows the filament to contract up to n\ 10^12 cm^-3. After the density reaches n\ 10^12 cm^-3, the filament becomes optically thick to H2 lines and the radial contraction subsequently almost stops. At this final hydrostatic stage, the fragment mass is lowered down to \ 1M_\ because of the high density of the filament. The dependence of the fragment mass upon the initial density could be translated into the dependence on the local amplitude of random Gaussian density fields or the epoch of the collapse of a parent cloud. Hence, it is predicted that the initial mass function of Population III stars is likely to be bimodal with peaks of \ 10² M_\ and \ 1M_\, where the relative heights could be a function of the collapse epoch.