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B. J. Mochejska (Copernicus Astronomical Center), L. M. Macri, D. D. Sasselov, K. Z. Stanek (Harvard-Smithsonian CfA)
The DIRECT project obtained about 170 nights on the FLWO 1.2-meter telescope and 35 nights on the MDM 1.3-meter telescope between 1996 and 1999 to search for detached eclipsing binaries and Cepheids in the M31 and the M33 galaxies. This was the first step in an ongoing program to improve the direct distance estimate to two important galaxies in the cosmological distance ladder -- M31 and M33. The massive photometry we have obtained as part of the DIRECT project over the past four years provides us with very good lightcurves for known and new Cepheid variables.
We investigate the influence of blending on the Cepheid distance scale. Blending leads to systematically low distances to galaxies observed with the HST, and therefore to systematically high estimates of H0. High-resolution HST images are compared to our ground-based data, obtained as part of the DIRECT project, for a sample of 22 Cepheids in M31 and 90 in M33. The average (median) V-band flux contribution from luminous companions which are not resolved on the ground-based images is about 19% (12%) of the flux of the Cepheid in M31 and 27% (17%) in M33. The average (median) I-band flux contribution is about 34% (24%) in M33.
Our ground-based resolution in M31 and M33 corresponds to the HST resolution at about 10 Mpc. When we use our M31 data to estimate the blending effects for a galaxy at ~25 Mpc observed by HST, we find that 50% of our Cepheids would have blended companions contributing more than 40% of the flux of the Cepheid. This has strong and direct implications for the Cepheid distances to galaxies observed by the HST Key Project on the Extragalactic Distance Scale and other teams.
Support for this work was provided by NASA through Grant AR-08354.02-97A from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.
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The author(s) of this abstract have provided an email address for comments about the abstract: mochejsk@camk.edu.pl