The Extragalactic Distance Scale Key Project. IV. The Discovery of Cepheids and a New Distance to M100 Using the Hubble Space Telescope

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Abstract

We report on observations of Cepheids in the Virgo spiral galaxy M100, based on data obtained with the Wide Field and Planetary Camera 2 (WFPC2) on board the Hubble Space Telescope (HST). The observations are part of the HST Key Project on the Extragalactic Distance Scale, which aims to provide a measurement of the Hubble constant H_0_ with 10% accuracy. A total of 12 epochs were obtained using the F555W filter (transformed to Johnson V), and 4 epochs using the F814W filter (transformed to Cousins I). Photometry on the data was performed using two independent packages, DoPHOT and DAOPHOT II/ALLFRAME; a total of 52 Cepheids, with periods ranging from about 10 to 70 days, were identified based on both sets of photometry. We have fitted V and I period-luminosity relations and derived apparent V and I distance moduli assuming a Large Magellanic Cloud distance modulus and mean color excess of μ_LMC_ = 18.50 +/- 0.10 mag and E(B - V) = 0.10 mag, respectively. Using the extinction law given by Cardelli, Clayton, & Mathis, and adopting our most recent WFPC2 ("long- exposure ) zero-point calibration, we obtain a true distance modulus for M100 of μ_0_ = 30.04 +/- 0.17 mag, corresponding to a distance d = 16.1 +/- 1.3 Mpc, and a total (Galactic plus internal) mean color excess E(B - V) = 0.10 +/- 0.06 mag. When compared with the preliminary distance of 17.1 +/- 1.8 Mpc derived by Freedman et al. from the same set of data, the results presented here benefit from a larger sample size, refinements in the zero-point calibration, and improvements in the software packages used for the photometry. The two results agree to within their quoted errors.

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