Reionization in the Light of Dark Stars
  We investigate the effect of Dark Stars (DSs) on the reionization history of
the Universe, and the interplay between them and feedback due to Lyman-Werner
(LW) radiation in reducing the Cosmic Microwave Background (CMB) optical depth
to a value within the $\tau = 0.054 \pm 0.007$ range measured by Planck. We use
a semi-analytic approach to evaluate reionization histories and CMB optical
depths, which includes Population II (Pop II) stars in atomic cooling halos and
Pop III stars in minihalos with LW feedback, preceded by a DS phase. We show
that while LW feedback by itself can reduce the integrated optical depth to the
last scattering surface to $\sim 0.05$ only if the Pop III star formation
efficiency is less than $\sim 0.2\%$, the inclusion of a population of DSs can
naturally lead to the measured CMB optical depth for much larger Pop III star
formation efficiencies $\gtrsim 1\%$.
