  We perform a detailed and systematic investigation of the possible impacts of
dark stars upon the reionization history of the Universe, and its signatures in
the cosmic microwave background (CMB). We compute hydrogen reionization
histories, CMB optical depths and anisotropy power spectra for a range of
stellar populations including dark stars. If dark stars capture large amounts
of dark matter via nuclear scattering, reionization can be substantially
delayed, leading to decreases in the integrated optical depth to last
scattering and large-scale power in the EE polarization power spectrum. Using
the integrated optical depth observed by WMAP7, in our canonical reionization
model we rule out the section of parameter space where dark stars with high
scattering-induced capture rates tie up more than ~90% of all the first
star-forming baryons, and live for over ~250 Myr. When nuclear scattering
delivers only moderate amounts of dark matter, reionization can instead be sped
up slightly, modestly increasing the CMB optical depth. If dark stars do not
obtain any dark matter via nuclear scattering, effects upon reionization and
the CMB are negligible. The effects of dark stars upon reionization and its CMB
markers can be largely mimicked or compensated for by changes in the existing
parameters of reionization models, making dark stars difficult to disentangle
from astrophysical uncertainties, but also widening the range of standard
parameters in reionization models that can be made consistent with
observations.
