2011ApJ...743..176G

We identify and phase a sample of 107 Cepheids with 10 days < P < 100 days in M81 using the Large Binocular Telescope and calibrate their B, V, and I mean magnitudes with archival Hubble Space Telescope (HST) data. The use of a ground-based telescope to identify and phase the Cepheids and HST only for the final calibration reduces the demand on this highly oversubscribed spacecraft by nearly an order of magnitude and yields period-luminosity (PL) relations with dispersions comparable to the best LMC samples. We fit the sample using the OGLE-II LMC PL relations and are unable to find a self-consistent distance for different band combinations or radial locations within M81. We can do so after adding a radial dependence to the PL zero point that corresponds to a luminosity dependence on metallicity of γ_µ= -0.56±0.36 mag/dex. We find marginal evidence for a shift in color as a function of metallicity, distinguishable from the effects of extinction, of γ₂= +0.07±0.03 mag/dex. We find a distance modulus for M81, relative to the LMC, of µ_M81- µ_LMC = 9.39±0.14 mag, including uncertainties due to the metallicity corrections. This corresponds to a distance to M81 of 3.6±0.2 Mpc, assuming an LMC distance modulus of 18.41 mag. We carry out a joint analysis of M81 and NGC 4258 Cepheids and simultaneously solve for the distance of M81 relative to NGC 4258 and the metallicity corrections. Given the current data, the uncertainties of such joint fits are dominated by the relative metallicities and the abundance gradients rather than by measurement errors of the Cepheid magnitudes or colors. We find µ_M81- µ_LMC= 9.40^+0.15_–0.11mag, µ_N4258 - µ_LMC= 11.08^+0.21_–0.17mag, and µ_N4258- µ_M81= 1.68 ±0.08 mag and metallicity effects on luminosity and color of γ_µ = -0.62^+0.31_–0.35 mag/dex and γ₂= 0.01±0.01 mag/dex. Quantitative analyses of Cepheid distances must take into account both the metallicity dependencies of the Cepheids and the uncertainties in the abundance estimates.