Properties of WNh stars in the Small Magellanic Cloud: evidence for homogeneous evolution

TL;DR

This study analyzes three WNh stars in the Small Magellanic Cloud to understand their properties, evolution, and wind clumping at low metallicity, revealing evidence for homogeneous evolution in one star and similar wind clumping to Galactic stars.

Contribution

It provides detailed atmospheric modeling of low-metallicity WNh stars, highlighting evidence for chemically homogeneous evolution and metallicity-independent wind clumping.

Findings

One star likely undergoing homogeneous evolution.

Clumping factor similar to Galactic Wolf-Rayet stars.

Evidence of CNO processing in all stars.

Abstract

We derive the physical properties of three WNh stars in the SMC to constrain stellar evolution beyond the main sequence at low metallicity and to investigate the metallicity dependence of the clumping properties of massive stars. We compute atmosphere models to derive the stellar and wind properties of the three WNh targets. A FUV/UV/optical/near-infrared analysis gives access to temperatures, luminosities, mass loss rates, terminal velocities and stellar abundances. All stars still have a large hydrogen mass fraction in their atmosphere, and show clear signs of CNO processing in their surface abundances. One of the targets can be accounted for by normal stellar evolution. It is a star with initial mass around 40-50 Msun in, or close to, the core He burning phase. The other two objects must follow a peculiar evolution, governed by fast rotation. In particular, one object is likely evolving homogeneously due to its position blue-ward of the main sequence and its high H mass fraction. The clumping factor of one star is found to be 0.15+/-0.05. This is comparable to values found for Galactic Wolf-Rayet stars, indicating that within the uncertainties, the clumping factor does not seem to depend on metallicity.