On the nature of massive helium star winds and Wolf-Rayet-type mass loss

Abstract

The mass-loss rates of massive helium stars are one of the major uncertainties in modern astrophysics. Regardless of whether they were stripped by a binary companion or managed to peel off their outer layers by themselves, the influence and final fate of helium stars -- in particular the resulting black hole mass -- highly depends on their wind mass loss as stripped-envelope objects. While empirical mass-loss constraints for massive helium stars have improved over the last decades, the resulting recipes are limited to metallicities with the observational ability to sufficiently resolve individual stars. Yet, theoretical efforts have been hampered by the complexity of Wolf-Rayet (WR) winds arising from the more massive helium stars. In an unprecedented effort, we calculate next-generation stellar atmosphere models resembling massive helium main sequence stars with Fe-bump driven winds up to $500\,M_\odot$ over a wide metallicity range between $2.0$ and $0.02\,Z_\odot$. We uncover a complex $Γ_\text{e}$-dependency of WR-type winds and their metallicity-dependent breakdown. The latter can be related to the onset of multiple scattering, requiring higher $L/M$-ratios at lower metallicity. Based on our findings, we derive the first ever theoretically-motivated mass-loss recipe for massive helium stars. We also provide estimates for LyC and He II ionizing fluxes, finding stripped helium stars to contribute considerably at low metallicity. In sharp contrast to OB-star winds, the mass loss for helium stars scales with the terminal velocity. While limited to the helium main sequence, our study marks a major step towards a better theoretical understanding of helium star evolution.