  We investigate the integrated properties of massive (>10 Msun), rotating,
single-star stellar populations for a variety of initial rotation rates
(v/vcrit=0.0, 0.2, 0.4, 0.5, and 0.6). We couple the new MESA Isochrone and
Stellar Tracks (MIST) models to the Flexible Stellar Population Synthesis
(FSPS) package, extending the stellar population synthesis models to include
the contributions from very massive stars (>100 Msun), which can be significant
in the first ~4 Myr after a starburst. These models predict ionizing
luminosities that are consistent with recent observations of young nuclear star
clusters. We also construct composite stellar populations assuming a
distribution of initial rotation rates. Even in low-metallicity environments
where rotation has a significant effect on the evolution of massive stars, we
find that stellar population models require a significant contribution from
fast-rotating (v/vcrit>0.4) stars in order to sustain the production of
ionizing photons beyond a few Myr following a starburst. These results have
potentially important implications for cosmic reionization by massive stars and
the interpretation of nebular emission lines in high-redshift star-forming
galaxies.
