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

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.