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

Abstract

We report on the identification of 255 candidate variable stars in a field located some 17 from the 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 Telescope. Photometric measurements in the F555W and F814W filters—analyzed independently with the DAOPHOT/ALLFRAME and DoPHOT software suites—have 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 10-48 days indicate a reddening-corrected mean distance modulus (m - M)₀ = 29.05 ± 0.14 (if the true modulus of the Large Magellanic Cloud is 18.50 ± 0.10, and if there 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.