The Extragalactic Distance Scale Key Project. XVI. Cepheid Variables in an Inner Field of M101

Abstract

We report on the identication of 255 candidate variable stars in a eld located some from the 1@ .7 center of the late-type spiral galaxy M101 \ NGC 5457, based on observations made with the Wide Field and Planetary Camera 2 on board the Hubble Space T elescope. Photometric measurements in the F555W and F814W ltersanalyzed independently with the DAOPHOT/ALLFRAME and DoPHOT software suiteshave been transformed to the Johnson V and Kron-Cousins I standard magnitude systems. Periods and intensity-averaged mean magnitudes for 61 carefully selected candidate Cepheid variables with periods in the range 1048 days indicate a reddening-corrected mean distance modulus (if the true modulus of the Large Magellanic Cloud is 18.50 ^0.10, and if there (m [ M) 0 \ 29.05 ^0.14 is no dependence of the period-luminosity relation on metal abundance) ; results consistent with this are obtained whether or not the sample is expanded to include a larger fraction of the candidates. Applying a metallicity-dependent correction of ]0.16 ^0.10 mag would increase this estimate to (m [ M) 0 \ 29.21 ^0.17 mag.