Evolution of stars with 60 and 200 Msun: predictions for WNh stars in the Milky Way

TL;DR

This study models the evolution of 60 and 200 solar mass stars at solar metallicity, focusing on wind prescriptions and their impact on stellar properties, providing insights into the formation and characteristics of WNh stars in the Milky Way.

Contribution

It introduces a wind transition model based on the Eddington factor and compares two stellar evolution codes, enhancing understanding of massive star evolution and WNh star formation.

Findings

WNh stars with high hydrogen surface fractions can form from initial masses over 60 M_sun.

The models reproduce observed properties of Galactic WNh stars during the main sequence.

Differences between the evolution codes are minimal at very high masses due to wind dominance.

Abstract

We study in detail the evolution of two massive stars at solar metallicity ($Z=0.014$), by calculating their final masses, radial expansion, and chemical enrichment. We run evolutionary models for initial masses 60 and 200 $M_\odot$, using MESA and the Geneva-evolution-code (GENEC). For the mass loss, we adopt the self-consistent m-CAK prescription for the optically thin winds of OB-type stars, a semi-empirical formula for H-rich optically thick wind of WNh stars, and a hydrodynamically consistent formula for the H-poor thick wind of classical Wolf-Rayet stars. The transition from thin to thick winds is set at $\Gamma_\text{e}=0.5$. For the 60 $M_\odot$ case, the GENEC model predicts a more efficient rotational mixing and more chemically homogeneous evolution, whereas the MESA model predicts a large radial expansion reaching the LBV phase. For the 200 $M_\odot$ case, differences between both evolution codes are less relevant because their evolution is dominated by wind mass loss with a weaker dependence on internal mixing. The switch of the mass-loss prescription based on the Eddington factor instead of the removal of outer layers, implies the existence of WNh stars with a large mass fraction of hydrogen at the surface ($X_\text{surf}\ge0.3$) formed from initial masses of $\gtrsim60$ $M_\odot$. These stars are constrained in a $T_\text{eff}$ range of the HRD which corresponds to the MS band, in agreement with the observations of Galactic WNh stars at $Z=0.014$. While our models employ a fixed $\Gamma_\text{e,trans}$ threshold for the switch to thick winds, rather than a continuous thin-to-thick wind model, the good reproduction of observations during the MS supports the robustness of the wind model upgrades, allowing its application to studies of late-stage stellar evolution before core collapse.