OPACITY, METALLICITY, AND CEPHEID PERIOD RATIOS IN THE GALAXY AND MAGELLANIC CLOUDS

Linear pulsation calculations are employed to reproduce the bump Cephe id resonance (P2/P0 = 0.5 at P0 congruent-to 10 days) and to model, in dividually, the P1/P0 period ratios for the dozen known Galactic beat Cepheids. Convection is ignored. The results point to a range of metal licity among the Cepheids, perhaps as large as 0.01 less than or simil ar to Z less than or similar to 0.02, with no evidence for any star ex ceeding Z = 0.02. We find masses and luminosities which range from M l ess than or similar to 4 M., log L less than or similar to 3.0 at P0 c ongruent-to 3 days to M less than or similar to 6 MD, log L less than or similar to 3.5 at P0 congruent-to 10 days. Similar parameters are i ndicated for the P0 congruent-to 10 days Cepheids in the LMC and SMC, provided that the resonance for these stars occurs at a slightly longe r period, P0 less than or similar to 11 days, as has been suggested in the literature. Our calculations were performed mainly using OPAL opa cities, but also with new opacities from the Opacity Project (OP). Onl y small differences were found between the OPAL results and,those from OP. Finally, some suggestions are made for possible future work, incl uding evolution and pulsation calculations, and more precise observati ons of Cepheids in the Magellanic Clouds.