The occurrence and evolution of Wolf-Rayet stars in different environments

Classical Wolf-Rayet (WR) stars are hydrogen-poor massive stars with prominent emission lines. They are therefore often used as benchmarks for massive helium (He) stars in stellar populations. However, their occurrence depends strongly on their environment, in particular on the metallicity (Z). Until now, our theoretical understanding of their feedback is rather fragmentary, hampering robust predictions about stellar populations and the occurrence of WR-type mass loss.
To develop a fundamental understanding of mass loss in massive He stars, we employ a new generation of model atmospheres including a consistent solution of the wind hydrodynamics. This way, we can study the ingredients of He-star winds and unveil the nature of WR-type mass loss. Our results reveal a complex picture with strong, non-linear dependencies on the luminosity-to-mass ratio and Z. Moreover, they provide a theoretical motivation for a population of He stars at low Z, which cannot be detected via WR-type spectral features.
The talk will present the results from our groundbreaking study of massive He-star atmosphere models, yielding the very first mass-loss recipe derived from first principles in this regime. Moreover, we will discuss first efforts to compare the impact of the new mass-loss recipe to typical implementations of WR mass loss in current evolution models. Performing our own studies, we will highlight the consequences of an improved treatment. We finish by outlining observational implications of our study including the need to correctly understand the manifold impacts of the presence and absence of WR-type mass-loss in the Universe.