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

Linear pulsation calculations are employed to reproduce the bump Cepheid resonance (P2/P0 = 0.5 at P0 ~ 10 days) and to model, individually, the P1/P0 period ratios for the dozen known Galactic beat Cepheids. Convection is ignored. The results point to a range of metallicity among the Cepheids, perhaps as large as 0.01 \u3c Z \u3c 0.02, with no evidence for any star exceeding Z = 0.02. We find masses and luminosities which range from M \u3c 4 M ʘ, log L \u3c 3.0 at P0 = 3 days to M \u3c 6 M ʘ, log L \u3e 3.5 at P0 = 10 days. Similar parameters are indicated for the P0 = 10 days Cepheids in the LMC and SMC, provided that the resonance for these stars occurs at a slightly longer period, P0 \u3e 11 days, as has been suggested in the literature. Our calculations were performed mainly using OPAL opacities, but also with new opacities from the Opacity Project (OP). Only small differences were found between the OPAL results and those from OP. Finally, some suggestions are made for possible future work, including evolution and pulsation calculations, and more precise observations of Cepheids in the Magellanic Clouds