HST obs. of NGC4258 Cepheids and HII regions

We expand on previous efforts and carry out a new Cepheid search in this system using the Hubble Space Telescope. We discover and measure a sample of 669 Cepheids in four new and two archival NGC 4258 fields, doubling the number of known Cepheids in this galaxy and obtaining an absolute calibration of their optical PLRs.

We determine a Wesenheit (W_VI_^HST^) PLR of -2.574(+/-0.034)-3.294(+/-0.042)logP, consistent with an independent Large Magellanic Cloud (LMC) calibration at the level of 0.032+/-0.044mag in its zero-point, after accounting for a metallicity dependence of -0.20+/-0.05mag/dex. Our determination of the PLR slope also agrees with the LMC-based value within their uncertainties. We attempt to characterize the metallicity effect of Cepheid PLRs using only the NGC 4258 sample, but a relatively narrow span of abundances limits our sensitivity and yields a W_VI_^HST^ zero-point dependence of -0.07+/-0.21mag/dex.