Detection of Beat Cepheids in M33 and Their Use as a Probe of the M33 Metallicity Distribution

Our analysis of the Deep CFHT M33 variability survey database has uncovered 5 Beat Cepheids (BCs) that are pulsating in the fundamental and first overtone modes. With only the help of stellar pulsation theory and of mass–luminosity (M -L) relations, derived from evolutionary tracks, we can accurately determine the metallicities Z of these stars. The [O/H] metallicity gradient of −0.16 dex/kpc that is inferred from the M33 galacto-centric distances of these Cepheids and from their ’pulsation’ metallicities is in excellent agreement with the standard spectroscopic metallicity gradients that are determined from H II regions, early B supergiant stars and planetary nebulae. Beat Cepheids can thus provide an additional, independent probe of galactic metallicity distributions. Subject headings: (stars: variables:) Cepheids, stars: oscillations (including pulsations), galaxies: individual: M33, galaxies: abundances, galaxies: evolution