Distance Determination To The Andromeda Galaxy Using Variable Stars

Distance determinations to Local Group galaxies conform the basic rungs of the cosmological distance scale. The chief goal of our project is obtaining direct and accurate distance determinations to M 31 from two important stellar populations: eclipsing binaries and Cepheids. A variability survey in the North-Eastern quadrant of M 31 was performed (in B and V passbands) with the Isaac Newton Telescope (La Palma) to identify suitable targets. The resulting catalog of 3,964 variable stars contains 437 eclipsing binaries and 416 Cepheids with similar to 250 epochs per filter. The selection of the 68 Cepheids less affected by blending were selected to determine a distance to M 31 of (m - M)(0) = 24.32 +/- 0.12 mag. At the same time, the analysis of the eclipsing binary sample (with the acquisition of Gemini/GMOS spectroscopy) has provided two direct distance determinations to M 31 : (m - M)(0) = 24.44 +/- 0.12 mag and (m - M)(0) = 24.46 +/- 0.19 mag. All the obtained distances are in complete agreement and provide a direct and accurate distance determination to M 31. The combination of these results with additional data could well reduce the distance uncertainty to M 31 to better than 4%.