Population III Star Formation in a ΛWDM Universe

In this paper we examine aspects of primordial star formation in a gravitinowarm dark matter universe with a cosmological constant. We compare a set ofsimulations using a single cosmological realization but with a wide range ofwarm dark matter particle masses which have not yet been conclusively ruled outby observations. The addition of a warm dark matter component to the initialpower spectrum results in a delay in the collapse of high density gas at thecenter of the most massive halo in the simulation and, as a result, an increasein the virial mass of this halo at the onset of baryon collapse. Both of theseeffects become more pronounced as the warm dark matter particle mass becomessmaller. A cosmology using a gravitino warm dark matter power spectrum assuminga particle mass of m_{WDM} ~ 40keV is effectively indistinguishable from thecold dark matter case, whereas the m_{WDM} ~ 15 keV case delays star formationby approx. 10^8 years. There is remarkably little scatter between simulationsin the final properties of the primordial protostar which forms at the centerof the halo, possibly due to the overall low rate of halo mergers which is aresult of the WDM power spectrum. The detailed evolution of the collapsing halocore in two representative WDM cosmologies is described. At low densities(n_{b} <= 10^5 cm^{-3}), the evolution of the two calculations is qualitativelysimilar, but occurs on significantly different timescales, with the halo in thelower particle mass calculation taking much longer to evolve over the samedensity range and reach runaway collapse. Once the gas in the center of thehalo reaches relatively high densities (n_{b} >= 10^5 cm^{-3}) the overallevolution is essentially identical in the two calculations.Comment: 36 pages, 12 figures (3 color). Astrophysical Journal, accepte