Metal-poor single Wolf-Rayet stars: The interplay of optically thick winds and rotation

TL;DR

This study models the evolution of metal-poor single stars considering rotation and optically thick winds, showing they can form Wolf-Rayet stars at low metallicity, aligning with observations in the Small Magellanic Cloud.

Contribution

It introduces new stellar evolution models that incorporate optically thick winds and rotation, explaining the formation of Wolf-Rayet stars at low metallicity.

Findings

Fast rotation enables envelope shedding in metal-poor stars.

Simulated WR stars match observed properties in the SMC.

Results impact understanding of black hole formation and supernovae.

Abstract

The Small Magellanic Cloud (SMC) hosts 12 known Wolf-Rayet (WR) stars, seven of which are apparently single. Their formation is a challenge for current stellar evolution models because line-driven winds are generally assumed to be quenched at a metallicity of Z < 0.004. Here, we present a set of mesa models of single stars with zero-age main sequence masses of 20 - 80 Msun considering different initial rotation speeds ({\Omega} = 0 - 0.7 {\Omega}_c), metallicities (Z = 0.002 - 0.0045), and wind mass-loss models (optically thin and thick winds). We show that if we account for optically thick winds, fast rotating ({\Omega} = 0.6 {\Omega}_c) single metal-poor O-type stars (with M > 20 Msun) shed their envelope and become WR stars even at the low metallicity of the SMC. The luminosity, effective temperature, evolutionary timescale, surface abundance, and rotational velocity of our simulated WR stars are compatible to the WRs observed in the SMC. We speculate that this scenario can also alleviate the excess of giant stars across the Humphreys-Davidson limit. Our results have key implications for black hole masses, (pair instability) supernova explosions, and other observable signatures.