Forming the First Stars in the Universe: The Fragmentation of Primordial Gas

Abstract

In order to constrain the initial mass function of the first generation of stars (Population III), we investigate the fragmentation properties of metal-free gas in the context of a hierarchical model of structure formation. We investigate the evolution of an isolated 3 σ peak of mass 2 × 10⁶ M_☉ that collapses at z_coll 30 using smoothed particle hydrodynamics. We find that the gas dissipatively settles into a rotationally supported disk that has a very filamentary morphology. The gas in these filaments is Jeans unstable with M_J ~ 10³ M_☉. Fragmentation leads to the formation of high-density (n > 10⁸ cm^-3) clumps that subsequently grow in mass by accreting the surrounding gas and by merging with other clumps up to masses of ~10⁴ M_☉. This suggests that the very first stars were rather massive. We explore the complex dynamics of the merging and tidal disruption of these clumps by following their evolution over a few dynamical times.