Spectral models for binary products: Unifying Subdwarfs and Wolf-Rayet
  stars as a sequence of stripped-envelope stars
  Stars stripped of their envelope through interaction in a binary are
generally not considered when accounting for ionizing radiation from stellar
populations, despite the expectation that stripped stars emit hard ionizing
radiation, form frequently and live 10-100 times longer than single massive
stars. We compute the first grid of spectral models specially made for stars
stripped in binaries for a range of progenitor masses (2-20$M_{\odot}$) and
metallicities covering a wide range.
  For stripped stars with masses between 0.3-7$M_{\odot}$, we find high
effective temperatures (20-100 kK, increasing with mass), small radii
(0.2-1$R_{\odot}$) and high bolometric luminosities, comparable to that of
their progenitor before stripping. The spectra show a continuous sequence that
naturally bridges subdwarf-type stars and Wolf-Rayet like spectra. For
intermediate masses we find hybrid spectral classes showing a mixture of
absorption and emission lines. These appear for stars with mass loss rates of
$10^{-8}-10^{-6}M_{\odot}$/yr, which have semi-transparent atmospheres. At low
metallicity, substantial H-rich layers are left at the surface and we predict
spectra that resemble O-type stars instead. We obtain spectra undistinguishable
from subdwarfs for stripped stars with masses up to 1.7$M_{\odot}$, which
questions whether the widely adopted canonical value of 0.47$M_{\odot}$ is
uniformly valid.
  Increasing the observed sample of stripped stars will provide necessary tests
for the physics of interaction, internal mixing and stellar winds. We
investigate the feasibility to detect stripped stars next to an optically
bright companion and recommend searches for their UV excess and possible
emission lines, most notably HeII4686 in the optical and HeII1640 in the UV.
Our models are publicly available for further investigations or inclusion in
spectral synthesis simulations.
