Eclipsing binaries suitable for distance determination in the Andromeda galaxy

The Local Group galaxies constitute a fundamental step in the definition of cosmic distance scale. Therefore, obtaining accurate distance determinations to the galaxies in the Local Group, and notably to the Andromeda Galaxy (M31), is essential for determining the age and evolution of the Universe. With this ultimate goal in mind, we started a project of using eclipsing binaries as distance indicators to M31. Eclipsing binaries have been proved to yield direct and precise distances that are essentially assumption-free. To do so, high-quality photometric and spectroscopic data were needed. As a first step in the project, broad band photometry (in Johnson B and V) was obtained in a region (34'x34') in the north eastern quadrant of the galaxy over 5 years. The data, containing more than 250 observations per filter, were reduced by means of the so-called difference image analysis technique and the DAOPHOT program. A catalog with 236 238 objects with photometry in both B and V passbands was obtained. The catalog is the deepest (V < 25.5 mag) obtained so far in the studied region and it contains 3964 identified variable stars, with 437 eclipsing binaries and 416 Cepheids. The most suitable eclipsing binary candidates for distance determination were selected according to their brightness and from the modelling of the obtained light curves. The resulting sample includes 24 targets with photometric errors around 0.01 mag. Detailed analysis (including spectroscopy) of some 5-10 of these eclipsing systems should result in a distance determination to M31 with a relative uncertainty of 2-3% and essentially free of systematic errors, thus representing the most accurate and reliable determination to date.