The extragalactic distance scale key project .4. The discovery of Cepheids and a new distance to M100 using the Hubble space telescope

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 mu(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 mu(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.