The Cepheid instability strip and the calibration of the primary distance scale

This study examines the possibility of galaxy-to-galaxy differences in the long-period Cepheid distributions of external galaxies. A simple theoretical framework is created and linear pulsation calculations are performed to model these distributions. The sturdy nature of the Cepheid period-luminosity (P-L) relation is affirmed, but both analytic arguments and the linear model grids point to potential systematic errors reaching up to a few tenths of a magnitude if the Cepheids in the calibrating and target galaxies have different distributions. We also point out some difficulties posed for stellar pulsation and evolution theory by the long-period Cepheids we have studied: the theoretical blue edge seems too hot and/or the inferred masses too large to account for the observed stars. Preliminary observational evidence is presented which marginally indicates the existence of two somewhat different types of distribution of long-period Cepheids in external galaxies, but further data are needed before this can be confirmed