THE ASTROPHYSICAL JOURNAL, 463:26-59, 1996 May 20

    THE EXTRAGALACTIC DISTANCE SCALE KEY PROJECT. III. THE DISCOVERY OF
               CEPHEIDS AND A NEW DISTANCE TO M101 USING THE
                          HUBBLE SPACE TELESCOPE

      DANIEL D. KELSON,[1] GARTH D. ILLINGWORTH,[1] WENDY F. FREEDMAN,[2]
    JOHN A. GRAHAM,[3] ROBERT HILL,[2] BARRY F. MADORE,[4] ABHIJIT SAHA,[5]
     PETER B. STETSON,[6] ROBERT C. KENNICUTT Jr.,[7] JEREMY R. MOULD,[8]
  SHAUN M. HUGHES,[9] LAURA FERRARESE,[5,10] RANDY PHELPS,[2] ANNE TURNER,[7]
 KEM H. COOK,[11] HOLLAND FORD,[5,10] JOHN G. HOESSEL,[12] AND JOHN HUCHRA[13]


                                   ABSTRACT

        We report on the discovery of 29 Cepheid variables in the galaxy
     M101 using the original Wide Field Camera (WFC) and the new Wide Field
     and Planetary Camera 2 (WFPC2) on the Hubble Space Telescope.  We
     observed a field in M101 at 17 independent epochs in V (F555W), five
     epochs in I (F785LP/F814W), and one epoch in B (F439W), with a time
     interval baseline of 381 days.  We have found Cepheids with periods
     ranging from 10 to 60 days.  The data have been calibrated using WFPC2
     observations with zero points derived from Omega Cen, Pal 4, and NGC
     2419 observations.  This calibration has been verified by using the
     Medium Deep Survey (MDS) WFC photometric zero points, and ground-based
     secondary standards in V and I.  The V calibrations agree to +/- 0.06
     mag, and the I calibrations agree to +/- 0.4 mag.  We have constructed
     V and I period-luminosity (PL) relations and have derived apparent
     distance moduli based on a distance modulus for the Large Magellanic
     Cloud (LMC) of 18.50 mag and a reddening of E(B-V) = 0.10 mag to the
     LMC Cepheids.  Period-residual minimization was used to minimize the
     effects of Malmquist bias on the period-luminosity relation fitting
     process.  Using a Galactic extinction law and the apparent V and I
     distance moduli, we have found a mean reddening for the M101 sample of
     E(B-V) = 0.03 mag and a true distance modulus to M101 of 29.34 +/- 0.17
     mag, corresponding to a distance of 7.4 +/- 0.6 Mpc.  The sources of
     error have been rigorously tracked through an error budget; systematic
     and random errors contribute roughly equally to the quoted error.  The
     mean gas-phase metal abundances in the LMC and in the M101 outer field
     are similar so we expect metallicity effects to be minimal.  These
     Cepheids will be used in conjunction with results from a Key Project
     search for Cepheids in an inner field, where the metallicity is larger
     by a factor of 5, to probe the effects of abundance on the Cepheid
     period-luminosity relation.

     Subject headings: Cepheids -- distance scale -- galaxies: distances
                       and redshifts -- galaxies: individual (M101)

[1]  UCO/Lick Observatory, University of California, Santa Cruz, CA 95064
[2]  Carnegie Observatories, 813 Santa Barbara Street, Pasadena, CA 91101
[3]  Department of Terrestrial Magnetism, Carnegie Institution of
     Washington, 5241 Broad Branch Road NW, Washington, DC 20015
[4]  NASA/IPAC Extragalactic Database, Infrared Processing and Analysis
     Center, Jet Propulsion Laboratory, California Institute of Technology,
     Pasadena, CA 91125
[5]  Space Telescope Science Institute, Homewood Campus, Baltimore, MD
     21218
[6]  Dominion Astrophysical Observatory, National Research Council, 5071
     West Saanich Road, Victoria, BC, Canada V8X 4M6
[7]  Steward Observatory, University of Arizona, Tucson, AZ 85721
[8]  Mt. Stromlo and Siding Springs Observatories, Private Bag, Weston
     Creek Post Office ACT 2611, Australia
[9]  Royal Greenwich Observatory, Madingley Road, Cambridge, UK CB3 0EZ
[10] Department of Physics and Astronomy, Bloomberg 501, Johns Hopkins
     University, 3400 North Charles Street, Baltimore, MD 21218
[11] Lawrence Livermore National Laboratory, MS L-401, P.O. Box 808,
     Livermore, CA 94550
[12] Department of Astronomy, University of Wisconsin, 475 North Charter
     Street, Madison, WI 53706
[13] Harvard-Smithsonian, Center for Astrophysics, 60 Garden Street,
     Cambridge, MA 02138