Rotation speed of the first stars

We estimate the rotation speed of Population III (Pop III) stars within a minihalo at z similar to 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 1012 cm-3. Representing the growing hydrostatic cores with accreting sink particles, we measure the velocities and angular momenta of all particles that fall on to 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 (greater than or similar to 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.