Population III Star Formation in a ΛWDM Universe

In this paper we examine aspects of primordial star formation in a gravitino warm dark matter universe with a cosmological constant (ΛWDM). We compare a set of simulations using a single cosmological realization but with a wide range of WDM particle masses that have not yet been conclusively ruled out by observations. The addition of a WDM component to the initial power spectrum results in a delay in 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. Both of these effects become more pronounced as the WDM particle mass becomes smaller. A cosmology using a gravitino WDM power spectrum assuming a particle mass of m_WDM ≃ 40 keV is effectively indistinguishable from the cold dark matter case, whereas the m_WDM ≃ 15 keV case delays star formation in the parent halo by ≃10⁸ yr. There is remarkably little scatter between simulations in the final properties of the protostar that forms at the center of the halo. The detailed evolution of the collapsing halo core in two representative WDM cosmologies is described. At low densities (n_b ≲ 10⁵ cm^-3) the evolution of the two calculations is qualitatively similar, but occurs on significantly different timescales, with the halo in the lower particle mass calculation taking much longer to evolve over the same density range and reach runaway collapse. Once the gas in the center of the halo reaches relatively high densities (n_b ≳ 10⁵ cm^-3), the overall evolution is essentially identical in the two calculations.