The Formation of the First Stars Ii. Radiative Feedback Processes and Implications for the Initial Mass Function

We consider the radiative feedback processes that operate during the formation of the first stars, including the photodissociation of H2, Lyman-α radiation pressure, formation and expansion of an H II region, and disk photoevaporation. These processes may inhibit continued accretion once the stellar mass has reached a critical value, and we evaluate this mass separately for each process. Photodissociation of H2 in the local dark matter minihalo occurs relatively early in the growth of the protostar, but we argue this does not affect subsequent accretion since by this time the depth of the potential is large enough for accretion to be mediated by atomic cooling. However, neighboring starless minihalos can be affected. Ionization creates an H II region in the infalling envelope above and below the accretion disk. Lyman-α radiation pressure acting at the boundary of the H II region is effective at reversing infall from narrow polar directions when the star reaches ∼ 20− 30M⊙, but cannot prevent infall from other directions. Expansion of the H II region beyond the gravitational escape radius for ionized gas occurs at masses ∼ 50− 100M⊙, depending on the accretion rate and angular momentum of the inflow. However, again, accretion from the equatorial regions can continue since the neutral accretion disk has a finite thickness and shields a substantial fraction of the accretion envelope from direct ionizing flux. At higher stellar masses, ∼ 140M⊙ in the fiducial case, the combination of declining accretion rates and increasing photoevaporation-driven mass loss from the disk act to effectively halt the increase in the protostellar mass. We identify this process as the mechanism that terminates the growth of Population III stars (i.e., stars with primordial composition) that have not been affected by prior star formation (Population III.1 stars). We discuss the implications of our results for the initial mass function of these stars. Subject headings: stars: formation — early universe — cosmology: theory