Rotation Speed of the First Stars
  We estimate the rotation speed of Population III (Pop III) stars within a
minihalo at z ~ 20 using a smoothed particle hydrodynamics (SPH) simulation,
beginning from cosmological initial conditions. We follow the evolution of the
primordial gas up to densities of 10^12 cm^-3. Representing the growing
hydrostatic cores with accreting sink particles, we measure the velocities and
angular momenta of all particles that fall onto these protostellar regions.
This allows us to record the angular momentum of the sinks and estimate the
rotational velocity of the Pop III stars expected to form within them. The
rotation rate has important implications for the evolution of the star, the
fate encountered at the end of its life, and the potential for triggering a
gamma-ray burst (GRB). We find that there is sufficient angular momentum to
yield rapidly rotating stars (> 1000 km s^-1, or near break-up speeds). This
indicates that Pop III stars likely experienced strong rotational mixing,
impacting their structure and nucleosynthetic yields. A subset of them was also
likely to result in hypernova explosions, and possibly GRBs.
