A New Cepheid Distance to the Giant Spiral M101 Based On Image
  Subtraction of HST/ACS Observations
  We accurately determine a new Cepheid distance to M101 (NGC 5457) using
archival HST/ACS V and I time series photometry of two fields within the
galaxy. We make a slight modification to the ISIS image subtraction package to
obtain optimal differential light curves from HST data. We discovered 827
Cepheids with periods between 3 and 80 days, the largest extragalactic sample
of Cepheids observed with HST by a factor of 2. With this large Cepheid sample
we find that the relative distance of M101 from the LMC is delta {\mu}LMC =
10.63 \pm 0.04 (random) \pm 0.07 (systematic) mag. If we use the geometrically
determined maser distance to NGC 4258 as our distance anchor, the distance
modulus of M101 is {\mu}0 = 29.04 \pm 0.05 (random) \pm 0.18 (systematic) mag
or D = 6.4 \pm 0.2 (random) \pm 0.5 (systematic) Mpc. The uncertainty is
dominated by the maser distance estimate (\pm 0.15 mag), which should improve
over the next few years. We determine a steep metallicity dependence, gamma,
for our Cepheid sample through two methods, yielding gamma = -0.84 \pm 0.22
(random) \pm 0.07 (systematic) mag dex-1 and gamma = -0.72+0.20 -0.22 (random)
\pm 0.06 (systematic) mag dex-1. We see marginal evidence for variations in the
Wesenheit P-L relation slope as a function of deprojected galactocentric
radius. We also use the TRGB method to independently determine the distance
modulus to M101 of {\mu}0 = 29.05 \pm 0.06 (random) \pm 0.12 (systematic) mag.
