THE HUBBLE-SPACE-TELESCOPE EXTRAGALACTIC DISTANCE SCALE KEY PROJECT .1. THE DISCOVERY OF CEPHEIDS AND A NEW DISTANCE TO M81

We report on the discovery of 30 new Cepheids in the nearby galaxy M81 based on observations using the Hubble Space Telescope (HST). The per iods of these Cepheids lie in the range of 10-55 days, based on 18 ind ependent epochs using the HST wide-band F555W filter. The HST F555W an d F785LP data have been transformed to the Cousins standard V and I ma gnitude system using a ground-based calibration. Apparent period-lumin osity relations at V and I were constructed, from which apparent dista nce moduli were measured with respect to assumed values of mu0 = 18.50 mag and E(B - V) = 0.10 mag for the Large Magellanic Cloud. The diffe rence in the apparent V and I moduli yields a measure of the differenc e in the total mean extinction between the M81 and the LMC Cepheid sam ples. A low total mean extinction to the M81 sample of E(B - V) = 0.03 +/- 0.05 mag is obtained. The true distance modulus to M81 is determi ned to be 27.80 +/- 0.20 mag, corresponding to a distance of 3.63 +/- 0.34 Mpc. These data illustrate that with an optimal (power-law) sampl ing strategy, the HST provides a powerful tool for the discovery of ex tragalactic Cepheids and their application to the distance scale. M81 is the first calibrating galaxy in the target sample of the HST Key Pr oject on the Extragalactic Distance Scale, the ultimate aim of which i s to provide a value of the Hubble constant to 10% accuracy.