Rotation Speed of the First Stars

TL;DR

This study uses cosmological simulations to estimate the rotation speeds of the first stars, finding they likely rotated rapidly, which influences their evolution, explosion mechanisms, and potential to produce gamma-ray bursts.

Contribution

It provides the first detailed simulation-based estimate of Population III star rotation speeds, linking early universe conditions to stellar evolution outcomes.

Findings

Pop III stars likely had near break-up rotation speeds

Rapid rotation implies strong rotational mixing in these stars

Some Pop III stars could have resulted in hypernovae and gamma-ray bursts

Abstract

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.