The Massive Star Population in M101. II. Spatial Variations in the
  Recent Star Formation History
  We investigate the star formation history (SFH) as a function of radius in
M101 using archival HST/ACS photometry. We derive the SFH from the resolved
stellar populations in five 2' wide annuli. Binning the SFH into time frames
corresponding to stellar populations traced by H$\alpha$, far ultraviolet
(FUV), and near ultraviolet (NUV) emission, we find that the fraction of
stellar populations young enough to contribute in H$\alpha$ is 15%-35% in the
inner regions, compared to less than 5% in the outer regions. This provides a
sufficient explanation for the lack of H$\alpha$ emission at large radii. We
also model the blue to red supergiant ratio in our five annuli, examine the
effects that a metallicity gradient and variable SFH have on the predicted
ratios, and compare to the observed values. We find that the radial behavior of
our modeled blue to red supergiant ratios is highly sensitive to both spatial
variations in the SFH and metallicity. Incorporating the derived SFH into
modeled ratios, we find that we are able to reproduce the observed values at
large radii (low metallicity), but at small radii (high metallicity) the
modeled and observed ratios are discrepant.
