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B. J. Mochejska (Copernicus Astronomical Center), L. M. Macri, D. D. Sasselov, K. Z. Stanek (Harvard-Smithsonian Center for Astrophysics)
This is a part of 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 between 1996 and 1999 provides us with very good light curves 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 102 Cepheids in M33. The average (median) flux contribution from luminous companions which are not resolved on the ground-based images in the V, I, B bands (SV, SI, SB) is about 23% (13%), 28% (20%), 28% (15%) of the flux of the Cepheid in M33. For 64 Cepheids with periods in excess of 10 days the average (median) values of SV, SI, SB are 16% (7%), 23% (12%), 20% (10%).
Support for this work was provided by NASA through Grant AR-08741 from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.