The Metallicity Dependence of the Cepheid P-L Relation in M101
  The impact of metallicity on the Cepheid Period-Luminosity (P-L) relation is
investigated using HST ACS V and I images of M101. Variations in the
reddening-free Wesenheit parameter (W), which is employed as a proxy for
luminosity, are examined as a function of the radial distance from the center
of M101 (and thus metallicity). We determine that there is no dependence of the
slope on metallicity. However, the intercept is found to depend on metallicity
by -0.33 +/- 0.12 mag/dex and -0.71 +/- 0.17 mag/dex using 2 and 3 sigma
rejection criteria, respectively. Sigma-clipping impacts the derived
metallicity dependence, and the 2-sigma criterion applied likely mitigates
blending, particularly in the crowded inner regions of M101. A
metallicity-corrected distance modulus of 28.96 +/- 0.11 for M101 is obtained
from 619 Cepheids, a result that agrees with the recently determined SN Ia
distance. The metallicity effects described can be bypassed by working at near
and mid-infrared wavelengths (e.g., the Carnegie Hubble Program).
