DARK MATTER ANNIHILATION AND PRIMORDIAL STAR FORMATION

We investigate the effects of weakly interacting massive particle (WIMP) dark matter annihilation on the formation of Population III. 1 (Pop III. 1) stars. We consider the relative importance of cooling due to baryonic radiative processes and heating due to WIMP annihilation. We analyze the dark matter and gas profiles of several halos formed in cosmological-scale numerical simulations. The heating rate depends sensitively on the dark matter density profile, which we approximate with a power law rho(chi) proportional to r(-alpha chi), in the numerically unresolved inner regions of the halo. If we assume a self-similar structure so that alpha(chi) similar to 1.5 as measured on the resolved scales similar to 1 pc, then for a fiducial WIMP mass of 100 GeV, the heating rate is typically much smaller (< 10(-3)) than the cooling rate for densities up to n(H) = 10(17) cm(-3). In one case, where alpha(chi) = 1.65, the heating rate becomes similar to the cooling rate by a density of n(H) = 10(15) cm(-3). The dark matter density profile is expected to steepen in the central baryon-dominated region similar to 1 pc due to adiabatic contraction, and we observe this effect, though with low resolution, in our numerical models. From these we estimate alpha(chi) similar or equal to 2.0. Heating now dominates cooling above n(H) similar or equal to 10(14) cm(-3), in agreement with the study of Spolyar, Freese, & Gondolo. We expect that this leads to the formation of an equilibrium structure with baryonic and dark matter density distributions exhibiting a flattened central core. Examining such equilibria, we find that the total luminosities due to WIMP annihilation are relatively constant and similar to 10(3) L-circle dot, set by the radiative luminosity of the baryonic core. We discuss the implications for Pop III. 1 star formation, particularly the subsequent growth of the protostar. Even if the initial protostar fails to accumulate any additional dark matter, its contraction to the main sequence could be significantly delayed by WIMP annihilation heating, potentially raising the mass scale of Pop III. 1 stars to masses >> 100 M-circle dot.