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V. Bromm (Yale University)
To elucidate the nature of the very first stars, one has to investigate the fragmentation of primordial, metal-free gas. This first generation of stars, the so-called Population III, must have been responsible for the initial enrichment with heavy elements. Although the search for Population III stars has proved elusive so far, upcoming CMB anisotropy probes (MAP/Planck) will study their signature, and NGST might be able to directly image them. In my thesis, I explore the physics of primordial star formation by means of three-dimensional simulations of the DM and gas components, using Smoothed Particle Hydrodynamics. In the context of hierarchical models of cosmic structure formation, the first stars are expected to form in virialized DM halos of mass a few times 106M\odot at redshifts z~30 - 20. The gas dissipatively settles into the center of the DM potential, and forms a very filamentary, lumpy disk. At this stage, the gas has attained typical temperatures of 200 - 300 K, and densities of n~103-4 cm-3, which corresponds to a Jeans mass of ~103M\odot. This result is very robust, being determined by the microphysics of H2 cooling. The gravitationally unstable gas fragments into high-density clumps (n>108 cm-3) with masses close to MJ. Subsequently, these clumps experience a complex history of tidal disruption, competitive accretion, and merging with other clumps. To adequately catch this behavior, it is essential to follow the evolution over a few dynamical times. I discuss the sensitivity of the fragmentation properties to variations of the important parameters. These results suggest that the Population III initial mass function (IMF) might favor massive stars. Finally, I address the effect of adding a small amount of metals to the gas, bearing on the question of how present-day star formation differs from the primordial case, and on the value of the metallicity where this transition does occur.
Support from the NASA ATP grant NAG5-7074 is gratefully acknowledged.