Population III Star Formation in a ΛCDM Universe. I. The Effect of Formation Redshift and Environment on Protostellar Accretion Rate

We perform 12 extremely high resolution adaptive mesh refinement cosmological simulations of Population III star formation in a ΛCDM universe, varying the box size and large-scale structure, to understand systematic effects in the formation of primordial protostellar cores. We find results that are qualitatively similar to those of previous groups. We observe that in the absence of a photodissociating ultraviolet background, the threshold halo mass for formation of a Population III protostar does not evolve significantly with time in the redshift range studied (33 > z > 19) but exhibits substantial scatter (1.5 < M_vir/10⁵ M_☉ < 7) due to different halo assembly histories: halos that assembled more slowly develop cooling cores at lower mass than those that assemble more rapidly, in agreement with previous work. We do, however, observe significant evolution in the accretion rates of Population III protostars with redshift, with objects that form later having higher maximum accretion rates ( ≃ 10^-4 M_☉ yr^-1 at z = 33 and ≃10^-2 M_☉ yr^-1 at z = 20). This can be explained by considering the evolving virial properties of the halos with redshift and the physics of molecular hydrogen formation at low densities. Our result implies that the inferred mass distribution of Population III stars is broader than previously thought and may evolve with redshift. Finally, we observe that our collapsing protostellar cloud cores do not fragment, consistent with previous results, which suggests that Population III stars that form in halos of mass 10⁵-10⁶ M_☉ always form in isolation.