Cepheids are probably the most studied variable stars. Their large amplitudes and intrinsic luminosities make them easily detectable in most photometric variability surveys. In addition, their well-known period-luminosity (P-L) relationship has made these variable stars one of the main cornerstones in deriving extragalactic distances. The importance of Cepheids for distance determination stands in contrast with the relative lack of additional information on the specific characteristics of extragalactic Cepheids and the possible corrections because of their particular properties (i.e., metallicity). A clear example is the Andromeda galaxy (M 31), where the first identification of Cepheids was already performed by Hubble in 1929. After the observations of Baade & Swope in 1965, few efforts have been dedicated to further analyze the Cepheid population in M 31.

This trend has changed in recent years with the emergence of new observational capabilities. Several variability surveys have started to study the stellar content in M 31 and other Local Group galaxies, obtaining large samples of Cepheids with accurate photometry. The detailed study of the observed Cepheids has emphasized the importance of an issue that was usually overlooked in most photometric studies: the effect of blending. It has been proposed that the magnitude of Cepheids may be affected by the light of unresolved companion stars (i.e., blends). The effect of blending is somewhat different from crowding or confusion noise, since companion stars appear to be in the same point-like source. Therefore, even when achieving a perfect point-spread function modeling, blending could still be present. The effect can be the same as in spectroscopic binaries, where the individual components cannot usually be resolved from ground-based images.

With the goal of obtaining accurate distance determinations to the Andromeda Galaxy and determining the age and evolution of the Universe in mind, astronomers started a project of using eclipsing binaries as distance indicators to M 31. 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'×34') in the north eastern quadrant of the galaxy over 5 years. The data, containing more than 250 observations per filter, generated a catalog with 236,238 objects with photometry in both B and V passbands. This catalog is the deepest (V<25.5 mag) photometric survey 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 M 31 with a relative uncertainty of 2-3% and essentially free of systematic errors, thus representing the most accurate and reliable determination to date.

The resulting sample of 416 Cepheids is the most complete in M 31 and has almost the same period distribution as the David Dunlap Observatory sample in the Milky Way. The large number of epochs (~250 per filter) has permitted the characterisation of the pulsation modes of 356 Cepheids, with 281 of them pulsating in the fundamental mode and 75 in the first overtone. They find that the blending contribution is as important as the metallicity correction when computing Cepheid distance determinations to M 31 (~0.1 mag). Since large amplitude Cepheids are less affected by blending, they used those with an amplitude larger than 0.8 mag to derive a distance to M 31 of (m-M)₀=24.32±0.12 mag.

This INT Wide Field Camera composite image in B and V bands of the Andromeda Galaxy centered at R.A.=00h44m46s and Dec.=+41°38'20'' shows the field of view used in the present study. (Credit: F. Vilardell, I. Ribas and C. Jordi, final image preparation by N. Szymanek) [ JPEG | TIFF ].

References:

• F. Vilardell, I. Ribas and C. Jordi , 2006, "Eclipsing binaries suitable for distance determination in the Andromeda galaxy", A&A, 459, 321.
• F. Vilardell, C. Jordi, and I. Ribas, 2007, "A comprehensive study of Cepheid variables in the Andromeda galaxy - Period distribution, blending, and distance determination", A&A, 473, 847..