Population III Star Formation in a ΛCDM Universe. II. Effects of a Photodissociating Background

Abstract

We examine aspects of primordial star formation in the presence of a molecular hydrogen-dissociating ultraviolet background. We compare a set of AMR hydrodynamic cosmological simulations using a single cosmological realization, but with a range of ultraviolet background strengths in the Lyman-Werner band. This allows us to study the effects of Lyman-Werner radiation on suppressing H₂ cooling at low densities, as well as the high-density evolution of the collapsing cloud core in a self-consistent cosmological framework. We find that the addition of a photodissociating background results in a delay of the collapse of high-density gas at the center of the most massive halo in the simulation and, as a result, an increase in the virial mass of this halo at the onset of baryon collapse. We find that, contrary to previous results, Population III star formation is not suppressed for J₂₁ ≥ 0.1, but occurs even with backgrounds as high as J₂₁ = 1. We find that H₂ cooling leads to collapse despite the depressed core molecular hydrogen fractions due to the elevated H₂ cooling rates at T = 2–5 × 10³ K. We observe a relationship between the strength of the photodissociating background and the rate of accretion onto the evolving protostellar cloud core, with higher LW background fluxes resulting in higher accretion rates. Finally, we find that the collapsing cloud cores in our simulations do not fragment at densities below n ~ 10¹⁰ cm⁻³, regardless of the strength of the LW background, suggesting that Population III stars forming in halos with T_vir ~ 10⁴ K may still form in isolation.