On the nature of massive helium star winds and Wolf-Rayet-type mass loss
  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 $\Gamma_\text{e}$-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.
