A program to determine a direct and accurate distance to M31 from eclipsing binaries

The Andromeda Galaxy (M31) is potentially a crucial calibrator for the Cosmic Distance Scale, and thus for determining the age and evolution of the Universe. Yet, currently the M31 distance is known to no better than similar to15%. We have demonstrated in our work on the LMC distance that double-line eclipsing binaries can serve as excellent "standard candles." Distances derived from eclipsing binaries are basically geometric and essentially free from many assumptions and uncertainties that plague other less direct methods. We present the extension of our program of using eclipsing binaries as standard candles to determine an accurate distance to M31. The photometric observations that yield the light curves are underway with the Isaac Newton Telescope at the ING (La Palma). Spectrophotometric observations will be conducted with the HST, and large telescopes (for example Gemini or GTC) are necessary to acquire the spectra for the radial velocity curves because of the faintness of the target stars (V similar to 19-20). Based on our previous experience, we expect to reduce the uncertainty of the M31 distance to better than 5%, thereby firmly calibrating the Cosmic Distance Scale. (C) 2004 Elsevier B.V. All rights reserved.