A Study of Wolf-Rayet stars Formed via Chemically Homogeneous Evolution

TL;DR

This study uses stellar evolution simulations to explore how rotation and metallicity influence the formation of Wolf-Rayet stars, highlighting chemically homogeneous evolution at low metallicity as a key pathway.

Contribution

It demonstrates that rapid rotation at low metallicity can lead to Wolf-Rayet stars through chemically homogeneous evolution, providing explanations for observed WR stars in different metallicity environments.

Findings

Rapid rotation promotes chemically homogeneous evolution at low metallicity.

High metallicity reduces rotational mixing efficiency.

Single-star models explain low-metallicity WR stars but not all high-metallicity WR types.

Abstract

Using the stellar evolution code---Modules for Experiments in Stellar Astrophysics (MESA), we investigate the evolution of massive stars with different rotational velocities and metallicities towards Wolf-Rayet stars. In our simulations, the initial rotating velocities are taken as 0, 250, 500 and 650 km s$^{-1}$, and the metallicities equal to 0.02, 0.014, 0.008, 0.006, 0.004 and 0.002. We show our rapid rotation models in the HR diagram compared with the observations. We find that the rotational mixing is less efficient at high metallicity, and these stars become Wolf-Rayet (WR) stars when the helium in their center is ignited. However, rapid rotating massive stars at low metallicity can easily evolve into WR stars due to the rotation resulted in chemically homogeneous evolution. This can explain the origin of single WR stars in galaxy at low metallicity. In our models, the observed SMC WR stars are consistent with the single-star evolution models. However at higher metallicities our single-star evolution models can only explain the luminous, hydrogen-rich WN stars and O stars (which are classified as WR stars previously).