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We review theoretical models of Population III.1 star formation, focusing on the proto-stellar feedback processes that are expected to terminate accretion and thus set the mass of these stars. We discuss how dark matter annihilation may modify this standard feedback scenario. Then, under the assumption that dark matter annihilation is unimportant, we predict the mass of stars forming in 12 cosmological minihalos produced in independent numerical simulations. This allows us to make a simple estimate of the Pop III.1 initial mass function and how it may evolve with redshift. [ABSTRACT FROM AUTHOR]

We describe how star formation is expected to proceed in the early metal-free Universe, focusing on the very first generations of stars. We then discuss how the star formation process may change as the effects of metallicity, external radiative feedback, and magnetic and turbulent support of the gas become more important. The very first stars (Pop III. 1) have relatively simple initial conditions set by cosmology and the cooling properties of primordial gas. We describe the evolution of these stars as they grow in mass by accretion from their surrounding gas cores and how the accretion process is affected and eventually terminated by radiative feedback processes, especially H II region expansion and disk photoevaporation. The ability of the protostar and its disk to generate dynamically important magnetic fields is reviewed and their effects discussed. Pop III. 1 star formation is likely to produce massive (∼100–200M⊙) stars that then influence their surroundings via ionization, stellar winds, and supernovae. These processes heat, ionize and metal-enrich the gas, thus altering the initial conditions for the next generation of star formation. Stars formed from gas that has been altered significantly by radiative and/or mechanical feedback, but not by metal enrichment (Pop III.2) are expected to have significantly smaller masses than Pop III.1 stars because of more efficient cooling from enhanced HD production. Stars formed from gas that is metal-enriched to levels that affect the dynamics of the collapse (the first Pop II stars) are also expected to have relatively low masses. We briefly compare the above star formation scenarios to what is known about present-day star formation. [ABSTRACT FROM AUTHOR]

How massive were the first stars? This question is of fundamental importance for galaxy formation and cosmic reionization. Here we consider how protostellar feedback can limit the mass of a forming star. For this we must understand the rate at which primordial protostars accrete, how they and their feedback output evolve, and how this feedback interacts with the infalling matter. We describe the accretion rate with an "isentropic accretion" model: ...[sub *], is initially very large (0_03M[sub ⊙] yr[sup -1] when m, = 1M[sub ⊙]) and declines as m[sup -3/7, sub *]. Protostellar evolution is treated with a model that tracks the total energy of the star. A key difference compared to previous studies is allowance for rotation of the infalling envelope. This leads to photospheric conditions at the star and dramatic differences in the feedback. Two feedback mechanisms are considered: HII region breakout and radiation pressure from Lyman-α and FUV photons. Radiation pressure appears to be the dominant mechanism for suppressing infall, becoming dynamically important around 20 M[sub ⊙]. [ABSTRACT FROM AUTHOR]

Supernova (SN) 1998bw and gamma-ray burst (GRB) 980425 offer the first direct evidence that supernovae are the progenitors of some GRBs. However, this burst was unusually dim, smooth and soft compared to other bursts with known afterglows. Whether it should be considered a prototype for cosmological GRBs depends largely on whether the supernova explosion and burst were asymmetrical or can be modeled as spherical. We address this question by treating the acceleration of the supernova shock in the outermost layers of the stellar envelope, the transition to relativistic flow, and the subsequent expansion (and further acceleration) of the ejecta into the surrounding medium. We find that GRB 980425 could plausibly have been produced by a collision between the relativistic ejecta from SN 1998bw and the star's pre-supernova wind; the model requires no significant asymmetry. This event therefore belongs to a dim subclass of GRBs and is not a prototype for jet-like cosmological GRBs. [ABSTRACT FROM AUTHOR]

The spatiotemporal coincidence of supernova (SN) 1998bw and gamma-ray burst (GRB) 980425 and this supernova's unusual optical and radio properties have prompted many theoretical models that produce GRBs from supernovae. We review the salient features of our simple, spherical model in which an energetic supernova explosion shock accelerates a small fraction of the progenitor's stellar envelope to mildly relativistic velocities. This material carries sufficient energy to produce a weak GRB and a bright radio supernova through an external shock against a dense stellar wind. [ABSTRACT FROM AUTHOR]